System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocatio

Hi I got this error when i navigate to a page from a list view in xamarin forms . I can't find the actual problem can any one help he to find where the problem is happening from this report . am using MVVMLight
[0:] Error NavigateTo BenWorkTasksPage. Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidCastException: Specified cast is not valid. at Xamarin.Forms.Label.FontStructPropertyChanged (Xamarin.Forms.BindableObject bindable, System.Object oldValue, System.Object newValue) [0x00000] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindablePropertyContext context, System.Object value, Boolean currentlyApplying, SetValueFlags attributes, Boolean silent) [0x0010e] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, SetValueFlags attributes, SetValuePrivateFlags privateAttributes) [0x0014b] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, Boolean fromStyle, Boolean checkAccess) [0x0005f] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, Boolean fromStyle) [0x00000] in <filename unknown>:0 at Xamarin.Forms.Setter.Apply (Xamarin.Forms.BindableObject target, Boolean fromStyle) [0x000a4] in <filename unknown>:0 at Xamarin.Forms.Style.ApplyCore (Xamarin.Forms.BindableObject bindable, Xamarin.Forms.Style basedOn) [0x0001e] in <filename unknown>:0 at Xamarin.Forms.Style.Xamarin.Forms.IStyle.Apply (Xamarin.Forms.BindableObject bindable) [0x0003e] in <filename unknown>:0 at Xamarin.Forms.VisualElement+MergedStyle.SetStyle (IStyle implicitStyle, IStyle classStyle, IStyle style) [0x00111] in <filename unknown>:0 at Xamarin.Forms.VisualElement+MergedStyle.set_Style (IStyle value) [0x0000d] in <filename unknown>:0 at Xamarin.Forms.VisualElement+<>c.<.cctor>b__211_6 (Xamarin.Forms.BindableObject bindable, System.Object oldvalue, System.Object newvalue) [0x00000] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindablePropertyContext context, System.Object value, Boolean currentlyApplying, SetValueFlags attributes, Boolean silent) [0x0010e] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, SetValueFlags attributes, SetValuePrivateFlags privateAttributes) [0x0014b] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, Boolean fromStyle, Boolean checkAccess) [0x0005f] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value) [0x00000] in <filename unknown>:0 at IWSM.XamarinApp.Views.BenWorkTasksPage.InitializeComponent () [0x0af0c] in <filename unknown>:0 at IWSM.XamarinApp.Views.BenWorkTasksPage..ctor (IWSM.Shared.DataModels.WorkTaskDataModel wtdm) [0x00008] in D:\Shamnad_TFS_Workspace\IWSM.XamarinApp\IWSM.XamarinApp\IWSM.XamarinApp\Views\BenWorking\BenWorkTasksPage.xaml.cs:31 at (wrapper managed-to-native) System.Reflection.MonoCMethod:InternalInvoke (System.Reflection.MonoCMethod,object,object[],System.Exception&) at System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) [0x00002] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:644 --- End of inner exception stack trace --- at System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) [0x00016] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:650 at System.Reflection.MonoCMethod.DoInvoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00089] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:635 at System.Reflection.MonoCMethod.Invoke (BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:663 at System.Reflection.ConstructorInfo.Invoke (System.Object[] parameters) [0x00000] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/ConstructorInfo.cs:62 at IWSM.XamarinApp.Common.NavigationService+<NavigateTo>d__15.MoveNext () [0x00173] in D:\Shamnad_TFS_Workspace\IWSM.XamarinApp\IWSM.XamarinApp\IWSM.XamarinApp\Common\NavigationService.cs:115 [0:] OnItemTapped: IWSM.Shared.DataModels.WorkTaskDataModel [0:] Error NavigateTo BenWorkTasksPage. Exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidCastException: Specified cast is not valid. at Xamarin.Forms.Label.FontStructPropertyChanged (Xamarin.Forms.BindableObject bindable, System.Object oldValue, System.Object newValue) [0x00000] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindablePropertyContext context, System.Object value, Boolean currentlyApplying, SetValueFlags attributes, Boolean silent) [0x0010e] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, SetValueFlags attributes, SetValuePrivateFlags privateAttributes) [0x0014b] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, Boolean fromStyle, Boolean checkAccess) [0x0005f] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, Boolean fromStyle) [0x00000] in <filename unknown>:0 at Xamarin.Forms.Setter.Apply (Xamarin.Forms.BindableObject target, Boolean fromStyle) [0x000a4] in <filename unknown>:0 at Xamarin.Forms.Style.ApplyCore (Xamarin.Forms.BindableObject bindable, Xamarin.Forms.Style basedOn) [0x0001e] in <filename unknown>:0 at Xamarin.Forms.Style.Xamarin.Forms.IStyle.Apply (Xamarin.Forms.BindableObject bindable) [0x0003e] in <filename unknown>:0 at Xamarin.Forms.VisualElement+MergedStyle.SetStyle (IStyle implicitStyle, IStyle classStyle, IStyle style) [0x00111] in <filename unknown>:0 at Xamarin.Forms.VisualElement+MergedStyle.set_Style (IStyle value) [0x0000d] in <filename unknown>:0 at Xamarin.Forms.VisualElement+<>c.<.cctor>b__211_6 (Xamarin.Forms.BindableObject bindable, System.Object oldvalue, System.Object newvalue) [0x00000] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueActual (Xamarin.Forms.BindableProperty property, Xamarin.Forms.BindablePropertyContext context, System.Object value, Boolean currentlyApplying, SetValueFlags attributes, Boolean silent) [0x0010e] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, SetValueFlags attributes, SetValuePrivateFlags privateAttributes) [0x0014b] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, Boolean fromStyle, Boolean checkAccess) [0x0005f] in <filename unknown>:0 at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value) [0x00000] in <filename unknown>:0 at IWSM.XamarinApp.Views.BenWorkTasksPage.InitializeComponent () [0x0af0c] in <filename unknown>:0 at IWSM.XamarinApp.Views.BenWorkTasksPage..ctor (IWSM.Shared.DataModels.WorkTaskDataModel wtdm) [0x00008] in D:\Shamnad_TFS_Workspace\IWSM.XamarinApp\IWSM.XamarinApp\IWSM.XamarinApp\Views\BenWorking\BenWorkTasksPage.xaml.cs:31 at (wrapper managed-to-native) System.Reflection.MonoCMethod:InternalInvoke (System.Reflection.MonoCMethod,object,object[],System.Exception&) at System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) [0x00002] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:644 --- End of inner exception stack trace --- at System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) [0x00016] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:650 at System.Reflection.MonoCMethod.DoInvoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00089] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:635 at System.Reflection.MonoCMethod.Invoke (BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00000] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:663 at System.Reflection.ConstructorInfo.Invoke (System.Object[] parameters) [0x00000] in /Users/builder/data/lanes/3339/39ebb778/source/maccore/_build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/corlib/System.Reflection/ConstructorInfo.cs:62 at IWSM.XamarinApp.Common.NavigationService+<NavigateTo>d__15.MoveNext () [0x00173] in D:\Shamnad_TFS_Workspace\IWSM.XamarinApp\IWSM.XamarinApp\IWSM.XamarinApp\Common\NavigationService.cs:115