Fixed crash on move selection. 4.6

[igorkorsukov]

Resolves: #25814

[igorkorsukov]

The problem was that currElem could become null, and we called the method (currElem->findAncestor(ElementType::VBOX)) on a null pointer in while

[mathesoncalum]

Hmm, I already tried this in #26726 but apparently it didn't work (I could never repro locally)

[cbjeukendrup]

@mathesoncalum It looks like you only had the check that Igor added as an IF_ASSERT_FAILED, and not yet the other check that Igor added

[igorkorsukov]

Hmm, I already tried this in #26726 but apparently it didn't work (I could never repro locally)

    const auto nextNonVBox = [this, isLeftDirection](EngravingItem* currElem) -> EngravingItem* {
        if (!currElem) {
            return nullptr;
        }

The problem is not that it can be null when calling the function nextNonVBox, but that it can become null in the body of while. But while will stop only if const EngravingItem* vBox becomes null, there is no check that the currElem we call the method findAncestor can be null

[mathesoncalum]

@cbjeukendrup @igorkorsukov Ahh yes - I misread. Thanks!

[DmitryArefiev]

Tested #25814 on Win10, Mac13.7.2, LinuxUbuntu24.04.3 LTS - FIXED