Fix RemoveFrontend deadlock

Author: Trolldemorted

Signal-Windows.Lib/SignalLibHandle.cs

@@ -147,6 +147,11 @@ public bool AddFrontend(CoreDispatcher d, ISignalFrontend w)
         public void RemoveFrontend(CoreDispatcher d)
         {
             Logger.LogTrace("RemoveFrontend() locking");
+            if (SynchronizationContext.Current != null)
+            {
+                Logger.LogCritical("RemoveFrontend must not be called with a syncchronization context");
+                throw new InvalidOperationException();
+            }
             SemaphoreSlim.Wait(CancelSource.Token);
             Logger.LogTrace("RemoveFrontend() locked");
             Logger.LogInformation("Unregistering frontend of dispatcher {0}", d.GetHashCode());

Signal-Windows/App.xaml.cs

@@ -357,7 +357,7 @@ private void CurrView_Consolidated(ApplicationView sender, ApplicationViewConsol
         {
             sender.Consolidated -= CurrView_Consolidated;
             var signalWindowsFrontend = Views[sender.Id];
-            Handle.RemoveFrontend(signalWindowsFrontend.Dispatcher);
+            Task.Run(() => Handle.RemoveFrontend(signalWindowsFrontend.Dispatcher)).Wait();
             Views.Remove(sender.Id);
             if (sender.Id != MainViewId)
             {