NRE when using CloseAll

[Therzok]

Select 3 files in the solution pad then press enter to open all 3 documents. This won't reproduce if you open one by one. Then do Close All Documents. Observe NRE being logged.

Version: b6e60ac

ERROR [2019-02-19 21:01:04Z]: An unhandled exception has occurred. Terminating Visual Studio? False
System.NullReferenceException: Object reference not set to an instance of an object
  at MonoDevelop.Ide.Gui.SdiWorkspaceWindow.<SelectWindow>b__63_0 (System.Object o, System.EventArgs args) [0x0000d] in /Users/therzok/Work/md/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Gui/SdiWorkspaceWindow.cs:294
  at Gtk.Application+InvokeProxy.HandlerInternal (System.IntPtr data) [0x0001d] in /Users/builder/jenkins/workspace/build-package-osx-mono/2018-08/external/bockbuild/builds/gtk-sharp-None/gtk/Application.cs:199

[Therzok]

cc @sevoku is this fixed by #7200?

[sevoku]

Yes! https://github.com/mono/monodevelop/pull/7200/files#diff-8da6280a955ed74b6e47759bb5b27f1fR305 ensures that it's not null there.