Add missing null check for content frame in ExportAnnotations

The `store.defaultContentFrame` selector had an incorrect return type, `Frame`
instead of `Frame|null`, because `frames[0]` has the static type `Frame`, even
though this can actually return undefined at runtime. As a result, TypeScript
didn't catch a missing null check when generating the default filename for the
export.

Fixes #5818

src/sidebar/components/ShareDialog/ExportAnnotations.tsx

@@ -60,7 +60,7 @@ function ExportAnnotations({
   const defaultFilename = useMemo(
     () =>
       suggestedFilename({
-        documentMetadata: contentFrame.metadata,
+        documentMetadata: contentFrame?.metadata,
         groupName: group?.name,
       }),
     [contentFrame, group],

src/sidebar/store/modules/frames.ts

@@ -163,7 +163,16 @@ const mainFrame = createSelector(
  */
 const defaultContentFrame = createSelector(
   (state: State) => state.frames,
-  frames => firstFrameWithoutId(frames) ?? frames[0] ?? null,
+  frames => {
+    const mainFrame = firstFrameWithoutId(frames);
+    if (mainFrame) {
+      return mainFrame;
+    } else if (frames.length > 0) {
+      return frames[0];
+    } else {
+      return null;
+    }
+  },
 );
 
 function searchUrisForFrame(frame: Frame): string[] {