core: fix crash when connecting graphics tablet

[ethan2-0]

Fixes issue #1415, which is a crash when plugging in a graphics tablet, by adding a null check. It appears to be necessary to have a window open and XWayland running to trigger the bug.

I developed this against v0.7.2 but rebased to master for this pull request. I can switch to another branch as well - I'm a bit new to this whole open-source thing, so I'm not sure what's best to do.