fix(ui_firestore): don't setState after dispose when fetching a page

packages/firebase_ui_firestore/lib/src/query_builder.dart

@@ -164,9 +164,12 @@ class _FirestoreQueryBuilderState<Document>
       _snapshot = _snapshot.copyWith(isFetching: true);
     }
 
-    // Delaying the setState so that fetchNextpage can be used within a child's
-    // "build" – most commonly ListView's itemBuilder
-    Future.microtask(() => setState(() {}));
+    // Delaying the setState so that fetchMore can be used within a child's
+    // "build" – most commonly ListView's itemBuilder. That build can be in the
+    // frame that removes this widget, so it may be gone by now.
+    Future.microtask(() {
+      if (mounted) setState(() {});
+    });
 
     final expectedDocsCount =
         (_pageCount + 1) * widget.pageSize

packages/firebase_ui_firestore/test/query_builder_test.dart

@@ -274,6 +274,44 @@ void main() {
     expect(snapshot.hasData, isTrue);
     expect(snapshot.isFetching, isFalse);
   });
+
+  testWidgets('scrolled away in the frame that its builder calls fetchMore', (
+    tester,
+  ) async {
+    final controller = ScrollController();
+    addTearDown(controller.dispose);
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: ListView(
+          controller: controller,
+          cacheExtent: 0,
+          itemExtent: 1000,
+          children: [
+            FirestoreQueryBuilder<Json>(
+              query: query,
+              pageSize: 2,
+              builder: (context, s, _) {
+                if (s.hasMore) s.fetchMore();
+                return const SizedBox();
+              },
+            ),
+            const SizedBox(),
+            const SizedBox(),
+          ],
+        ),
+      ),
+    );
+
+    // The snapshot rebuilds the query builder, whose builder fetches the
+    // next page, and the list drops it while laying out that same frame. The
+    // setState that fetchMore delays runs after it is disposed.
+    query.emit(3, size: 3, fromCache: false);
+    controller.jumpTo(2000);
+    await tester.pump();
+    expect(tester.takeException(), isNull);
+  });
 }
 
 class FakeQuery extends Fake implements Query<Json> {