Steps to reproduce:
1. Download attached project
2. Build a UWP player (IL2CPP, x64, Debug)
3. When app launches click the button to open UWP pop up
4. Close pop up

Expected results: Second UWP pop up shows up
Actual results: Player crashes. In 2018.3.13f and 2017.4 app doesn't crash, but the second pop up doesn't show up

Reproduced with: 2019.2.0a6, 2019.1.0a5, 2018.3.7f1, 2017.4.21f1

Assertion failed on expression: 'CurrentThreadIsMainThread()'
UnityEngine.DebugLogHandler:Internal_LogException(Exception, Object)
UnityEngine.DebugLogHandler:LogException(Exception, Object)
UnityEngine.Logger:LogException(Exception, Object)
UnityEngine.Debug:LogException(Exception)
UnityEngine.UnhandledExceptionHandler:<RegisterUECatcher>m__0(Object, UnhandledExceptionEventArgs)
System.UnhandledExceptionEventHandler:Invoke(Object, UnhandledExceptionEventArgs)

Note:
- not reproducible on .NET

Workaround: after await, use UnityEngine.WSA.Application.InvokeOnUIThread() to get back onto the UI thread. Like so:

MessageDialog dialog1 = new MessageDialog("Dialog #1");
await dialog1.ShowAsync();

UnityEngine.WSA.Application.InvokeOnUIThread(async () =>
{
MessageDialog dialog2 = new MessageDialog("Dialog #2");
await dialog2.ShowAsync();
}, false);