fix: prevent crash in determineDropResult (#1281) (#1367)

When target is undefined, determineDropResult crashes. I was not able to determine why `registry.getTarget` would return undefined. The issues happens sometimes when I am dragging and dropping quickly. Perhaps the drop target is getting unmounted as mentioned in #31.

I am happy to dig deeper if someone can point me in the right direction regarding which files might be relevant.

This fixes #1281.

packages/dnd-core/src/actions/dragDrop/drop.ts

@@ -50,7 +50,7 @@ function determineDropResult(
 	monitor: DragDropMonitor,
 ) {
 	const target = registry.getTarget(targetId)
-	let dropResult = target.drop(monitor, targetId)
+	let dropResult = target ? target.drop(monitor, targetId) : undefined
 	verifyDropResultType(dropResult)
 	if (typeof dropResult === 'undefined') {
 		dropResult = index === 0 ? {} : monitor.getDropResult()