fix(dia.Paper): handle element removal during pointer events

[Geliogabalus]

Summary

• Fixes an issue where removing an element during pointermove or pointerup event handlers would cause errors
• When findViewByModel() returns null (because the model was removed), falls back to using the original view reference

Problem

When an element is removed from the graph during a pointer event handler (e.g., deleting an element after dragging), subsequent calls to view.pointermove() or view.pointerup() would fail because findViewByModel(view.model) returns undefined for removed models.

Solution

Added a fallback to use the original view reference when findViewByModel() returns falsy:

// Before
view = this.findViewByModel(view.model);

// After
view = this.findViewByModel(view.model) || view;

This change is applied to both pointermove and pointerup handlers in packages/joint-core/src/dia/Paper.mjs.

[kumilingus]

Can this be done instead?

simulate.mousedown({ el: testElementRect });
simulate.mousemove({ el: testElementRect });
testElement.remove();
simulate.mouseup({ el: testElementRect });

[Geliogabalus]

I checked, and with this setup it never even goes into paper.pointerup function when event is triggered, so this is meaningless I think.

[Geliogabalus]

I think it is important that at the start of the event chain, the view is in the dom as a part of the paper.