Fixes crash in v3.12.2 when opening the "Modify Table" dialog - #3075

In master was already fixed in b076211

I don't know if we want to do a bug fix release, so I've opened this PR for the v3.12.x branch.

My C++/Qt skills aren't good enough to review this any more, but the idea of doing this in the 3.12.x branch is a good one. 😄

My C++/Qt skills aren't good enough to review this any more, but the idea of doing this in the 3.12.x branch is a good one. smile

There's no need of review for the code, because the same change was applied by @MKleusberg in the master branch. The reason behind the pull request is that I didn't want to modify a release branch without someone else agreeing. It makes sense if we are going to make a fix release for this branch, but we might prefer to go directly for the long delayed 3.13 release.

Makes sense. Just merged this, so it's in the branch for whoever builds from this branch anyway. 😄