GUI: crash on units change in DepthProbe simulation

1. Create DepthProbe instrument
2. Create sample
3. Simulate
4. Properties panel --> change axes units --> CRASH

changed milestone to %21 GUI fixes

added GUI bug crash labels

assigned to @m.svechnikov

This happens because DepthProbeInstrumentItem::createCoordSystem() returns a nullpointer which is getting dereferenced in the next line inside the method GUI::Model::JobItemUtils::updateDataAxes when returning the converted axes.

I did make it work to not crash anymore. But still did not implement the TODO written in the method body, so I don't know if this fix is what we want there.

mentioned in merge request !1225 (merged)

mentioned in commit 065536ba

closed with merge request !1225 (merged)

mentioned in issue #480 (closed)

added release-critical label