message: fix(handle): check if source or target exist
diff: @@ -99,9 +99,10 @@ export function onMouseDown(
  ): void {
  const reactFlowNode = (event.target as Element).closest('.react-flow');
  const elementBelow = document.elementFromPoint(event.clientX, event.clientY);
  - const elementBelowIsTarget = elementBelow?.classList.contains('target') || false;
  + const elementBelowIsTarget = elementBelow?.classList.contains('target');
  + const elementBelowIsSource = elementBelow?.classList.contains('source');
  - if (!reactFlowNode) {
  + if (!reactFlowNode || (!elementBelowIsTarget && !elementBelowIsSource)) {
  return;
  }
  