fix crash on more 2D points than 3D points

Currently when you select and fetch 2D points without loading the 3D points and then try to visualize them, Petrack crashes. This MR adds a check to prevent this from happening. Plus I changed a return statement and translated the comment since it bothered me

Reviewer Checklist

General code quality

• naming conventions are met (see .clang-tidy for detailed information)
• no static analyzer warnings in new code parts (e.g., use clang-tidy for checking)

General usability

• old versions of pet-files are still loadable
• documentation updated (if applicable)

Only if changes in UI

• new elements are also saved and loaded from pet-file
• check if tab order is still correct
• all new SpinBoxes are promoted
• new keybindings added to Petrack::keyBindings()

Closes #438 (closed)