Default elementFromPoint implementation can result in uncaught exception through normal usage

[needsmorecoffee]

Sometimes the native function document.elementFromPoint(x, y); can return null under normal usage scenarios - e.g. if the element is dragged off-screen - as some browsers (Chrome for mobile is one I encountered issues with) will not return an element if the x, y coords are outside the current viewport.

However, the default elementFromPoint implementation included in the mobile-drag-drop library does not return null if the element returned by the native function is null, instead it returns undefined. This causes issues further down the line as there are null checks but not undefined checks.

Example:

Uncaught TypeError: Cannot read properties of undefined (reading 'getBoundingClientRect')
    at createDragEventFromTouch (drag-utils.ts:113:38)
    at dispatchDragEvent (drag-utils.ts:145:22)
    at DragOperationController._dragAndDropProcessModelIteration (drag-operation-controller.ts:514:21)
    at drag-operation-controller.ts:240:18

This function can be overwritten in the config so it's not a blocker but still might be worth looking at.

[needsmorecoffee]

Clone of #180 apparently does not occur in the latest 2.x branch