Also crashes when docking a window onto editor.

How to reproduce:
1. Open editor with -debugallocator flag (from command line)
2. Open profiler (or any other floating/not connected to editor window)
3. Close profiler

Actual behaviour: Editor crashes
Expected behaviour: Floating window is closed

Reproduced on: 2017.1.0b2, 2017.1.0b3
Windows only.

========== OUTPUTING STACK TRACE ==================

0x0000000140029D2B (Unity) GUIView::OnInputEvent
0x00000001401A0C13 (Unity) GUIView::ProcessInputEvent
0x0000000140006AF3 (Unity) GUIView::ProcessEventMessages
0x000000014002CD25 (Unity) GUIView::GUIViewWndProc
0x00007FF9C5221C24 (USER32) CallWindowProcW
0x00007FF9C522156C (USER32) DispatchMessageW
0x00000001419F31A9 (Unity) MainMessageLoop
0x00000001419F4BD7 (Unity) WinMain
0x0000000141C99DE8 (Unity) __tmainCRTStartup
0x00007FF9C35A8364 (KERNEL32) BaseThreadInitThunk
0x00007FF9C53E70D1 (ntdll) RtlUserThreadStart

========== END OF STACKTRACE ===========