Both VS 2015 sp3 and VS 2017 crashing when selecting item from navigation bar

Visual Studio 2015 AND 2017 are both crashing on one particular Web Forms solution (others are OK) when selecting an item in the navigation bar in a code behind page.

For example, I can select a button control, but then when I select the click event in the right hand drop down, VS crashes. This happens with all events in a code behind page and is not specific to buttons etc. As a note, if I open up the project file and not the solution it does the same thing, not sure if that matters.

I have tried cleaning the solution but I can't really think of much else to try. 

Again, this is only a problem with one solution, in any of the codebehind pages. 

Event viewer shows the following:

Application: devenv.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.ArgumentNullException
   at Microsoft.CodeAnalysis.Simplification.Simplifier+<ReduceAsync>d__11.MoveNext()

Exception Info: System.AggregateException
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean)
   at System.Threading.Tasks.Task.Wait(Int32, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.VisualBasic.NavigationBar.AbstractGenerateCodeItem+VB$StateMachine_6_GetGeneratedDocumentAsync.MoveNext()

Exception Info: System.AggregateException
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean)
   at System.Threading.Tasks.Task.Wait(Int32, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.VisualBasic.NavigationBar.VisualBasicNavigationBarItemService.GenerateCodeForItem(Microsoft.CodeAnalysis.Document, Microsoft.CodeAnalysis.Editor.VisualBasic.NavigationBar.AbstractGenerateCodeItem, Microsoft.VisualStudio.Text.Editor.ITextView, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.VisualBasic.NavigationBar.VisualBasicNavigationBarItemService.NavigateToItem(Microsoft.CodeAnalysis.Document, Microsoft.CodeAnalysis.Editor.NavigationBarItem, Microsoft.VisualStudio.Text.Editor.ITextView, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.Implementation.NavigationBar.NavigationBarController.NavigateToItem(Microsoft.CodeAnalysis.Editor.NavigationBarItem, Microsoft.CodeAnalysis.Document, Microsoft.VisualStudio.Text.ITextSnapshot, Microsoft.CodeAnalysis.Editor.INavigationBarItemService, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.Implementation.NavigationBar.NavigationBarController.ProcessItemSelectionSynchronously(Microsoft.CodeAnalysis.Editor.NavigationBarItem, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.Implementation.NavigationBar.NavigationBarController+<>c__DisplayClass22_0.<OnItemSelected>b__0(Microsoft.CodeAnalysis.Editor.Host.IWaitContext)
   at Microsoft.VisualStudio.LanguageServices.Implementation.Utilities.VisualStudioWaitIndicator.Wait(System.String, System.String, Boolean, Boolean, System.Action`1<Microsoft.CodeAnalysis.Editor.Host.IWaitContext>)
   at Microsoft.CodeAnalysis.Editor.Implementation.NavigationBar.NavigationBarController.OnItemSelected(System.Object, Microsoft.CodeAnalysis.Editor.NavigationBarItemSelectedEventArgs)
   at Microsoft.VisualStudio.LanguageServices.Implementation.NavigationBar.NavigationBarClient.Microsoft.VisualStudio.TextManager.Interop.IVsDropdownBarClient.OnItemChosen(Int32, Int32)
   at Microsoft.VisualStudio.Editor.Implementation.VsDropDownBarAdapterMargin.ChooseSelected(System.Windows.Controls.ComboBox)
   at Microsoft.VisualStudio.Editor.Implementation.VsDropDownBarAdapterMargin.comboBox_PreviewMouseUp(System.Object, System.Windows.Input.MouseButtonEventArgs)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(System.Delegate, System.Object)
   at System.Windows.RoutedEventArgs.InvokeHandler(System.Delegate, System.Object)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(System.Object, System.Windows.RoutedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(System.Object, System.Windows.RoutedEventArgs, Boolean)
   at System.Windows.UIElement.RaiseEventImpl(System.Windows.DependencyObject, System.Windows.RoutedEventArgs)
   at System.Windows.UIElement.RaiseTrustedEvent(System.Windows.RoutedEventArgs)
   at System.Windows.UIElement.RaiseEvent(System.Windows.RoutedEventArgs, Boolean)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(System.Windows.Input.InputEventArgs)
   at System.Windows.Input.InputProviderSite.ReportInput(System.Windows.Input.InputReport)
   at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr, System.Windows.Input.InputMode, Int32, System.Windows.Input.RawMouseActions, Int32, Int32, Int32)
   at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr, MS.Internal.Interop.WindowMessage, IntPtr, IntPtr, Boolean ByRef)
   at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
   at MS.Win32.HwndWrapper.WndProc(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(System.Object)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority, System.TimeSpan, System.Delegate, System.Object, Int32)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr, Int32, IntPtr, IntPtr)

Thanks in advance

Ben