Use NativeWindow.Exists instead of Visible to determine if program should exit main loop.

src/ImGui.NET.SampleProgram/SampleWindow.cs

@@ -157,7 +157,7 @@ namespace ImGuiNET
         public void RunWindowLoop()
         {
             _nativeWindow.Visible = true;
-            while (_nativeWindow.Visible)
+            while (_nativeWindow.Exists)
             {
                 _previousFrameStartTime = DateTime.UtcNow;