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]

the second test is not removing the element at all.