Fix error while deleting query history item

When deleting a query history item and the "next" query is still
running, the `completedQuery` is `undefined`. This commit fixes it by
using optional chaining to ensure that the `completedQuery` is defined
before accessing its `successful` property.

extensions/ql-vscode/src/interface.ts

@@ -399,7 +399,7 @@ export class ResultsView extends AbstractWebview<
     forceReveal: WebviewReveal,
     shouldKeepOldResultsWhileRendering = false,
   ): Promise<void> {
-    if (!fullQuery.completedQuery.successful) {
+    if (!fullQuery.completedQuery?.successful) {
       return;
     }