Ensure selected dialogue ID is set on initial load

src/app/App.tsx

@@ -2478,6 +2478,22 @@ export function App({ store, initialStatusMessage }: AppProps) {
     setPlayerStartKeyboardCaptureAction(null);
   }, [selectedPlayerStart?.id]);
 
+  useEffect(() => {
+    if (
+      selectedDialogueId !== null &&
+      editorState.projectDocument.dialogues.dialogues[selectedDialogueId] !==
+        undefined
+    ) {
+      return;
+    }
+
+    setSelectedDialogueId(projectDialogueList[0]?.id ?? null);
+  }, [
+    editorState.projectDocument.dialogues.dialogues,
+    projectDialogueList,
+    selectedDialogueId
+  ]);
+
   useEffect(() => {
     schedulePaneHeightRef.current = schedulePaneHeight;
   }, [schedulePaneHeight]);