PR: Fix segfault on shortcut preferences when using mouse

[csabella]

Fixes #5426.

To recreate the issue (at least on Ubuntu):

• Open Preferences and click on Keyboard Shortcuts.
• Select a shortcut to edit, either by using Enter on the keyboard or by double clicking the line (meaning that mouse vs. keyboard isn't important at this step).
• Without this change, if you immediately click Cancel, the seg fault occurs.
• However, if instead of clicking, you use the keyboard 'Tab' to highlight cancel (as the text in the dialog suggests to do to switch focus), then you can press Enter on the Cancel button and not get the seg fault.
• The same thing happens with the OK button.

This seemed to be a difference between keyboard entry and mouse clicking on the buttons. In the code, the only difference was setting widget focus, so by adding focus to the mouse click, it prevents the seg fault.

I don't know the underlying cause, so this may be a hack in order to fix it, but it does work.

[ccordoba12]

Thanks a lot for this @csabella! Could you move this PR to 3.x since you said this bug is also present there? Thanks!

[csabella]

I had done it under 3.x, but forgot to change it when creating the PR. Thanks for catching it and sorry about that!

[ccordoba12]

Really good to have a fix for this one! Thanks Cheryl!