wk crashes on right-click when no segmentation is shown

[jstriebel]

Context

When opening a dataset and right-clicking in a 2D view, wk crashes when no segmentation is shown (either because they are disabled or not segmentation is part of the dataset).

Expected Behavior

wk should not crash, segmentation specific functionality in the right-click menu should be omitted/disabled.

Current Behavior

wk crashes.

Steps to Reproduce the bug

1. open https://webknossos.org/datasets/scalable_minds/l4_sample/view#3584,3584,1024,0,1.3
2. disable the segmentation layer
3. right click anywhere in XY-port
4. wk crashes 😿