Crash on teardown when only part of the open canvases has been closed

The problem is the one described here:

https://root-forum.cern.ch/t/problems-while-exiting-root-q-with-canvas-3/39672/4

Essentially when there are many (for example two) canvases open, and one closes, via the red 'X' button, some but not all of them (only one, in the two total canvases example), when later one quits root (.q or Browser->Quit Root), there's a crash.

Closing the canvases using TCanvas::Close() seems not triggering the problem.

The minimal steps to reproduce the issue (confirmed by several people, as shown in the ROOT forum post) is to:

• open 2 canvases
• close the second via the red 'X' button
• close ROOT

root [0] TCanvas a;

root [1] TCanvas b;

>>>> close "b" via GUI ("close" red button on Mac)

root [2] .q

2020-06-10 08:50:34.240 root.exe[48047:6522471] Fatal error: requested non-existing drawable 99

2020-06-10 08:50:34.240 root.exe[48047:6522471] This window not found among allocated/deleted drawables

 *** Break *** segmentation violation

[/usr/lib/system/libsystem_platform.dylib] _sigtramp (no debug info)

[<unknown binary>] (no debug info)