Fix potential null exception while dragging cells

src/vs/workbench/contrib/notebook/browser/view/renderers/cellRenderer.ts

@@ -467,10 +467,12 @@ export class MarkdownCellRenderer extends AbstractCellRenderer implements IListR
 		DOM.reset(dragImageContainer, templateData.container.cloneNode(true));
 
 		// Remove all rendered content nodes after the
-		const markdownContent = dragImageContainer.querySelector('.cell.markdown')!;
-		const contentNodes = markdownContent.children[0].children;
-		for (let i = contentNodes.length - 1; i >= 1; i--) {
-			contentNodes.item(i)!.remove();
+		const markdownContent = dragImageContainer.querySelector('.cell.markdown');
+		const contentNodes = markdownContent?.children[0].children;
+		if (contentNodes) {
+			for (let i = contentNodes.length - 1; i >= 1; i--) {
+				contentNodes.item(i)!.remove();
+			}
 		}
 
 		return dragImageContainer;