fix: allow using MapController before the map has been laid out

lib/src/map/controller/map_controller_impl.dart

@@ -16,7 +16,14 @@ class MapControllerImpl extends ValueNotifier<_MapControllerState>
     implements MapController {
   final _mapEventStreamController = StreamController<MapEvent>.broadcast();
 
-  late MapInteractiveViewerState _interactiveViewerState;
+  /// The viewer of the map this controller is attached to, or `null` while it
+  /// does not exist yet.
+  ///
+  /// The viewer is created during layout, so a `FlutterMap` that has been
+  /// built but not laid out - for example one on a route that another route
+  /// covers from the first frame - has none, even though `onMapReady` has
+  /// already fired.
+  MapInteractiveViewerState? _interactiveViewerState;
 
   Animation<LatLng>? _moveAnimation;
   Animation<double>? _zoomAnimation;
@@ -336,7 +343,7 @@ class MapControllerImpl extends ValueNotifier<_MapControllerState>
 
     if (value.options != null &&
         value.options!.interactionOptions != newOptions.interactionOptions) {
-      _interactiveViewerState.updateGestures(
+      _interactiveViewerState?.updateGestures(
         value.options!.interactionOptions,
         newOptions.interactionOptions,
       );
@@ -702,7 +709,7 @@ class MapControllerImpl extends ValueNotifier<_MapControllerState>
 
   void _emitMapEvent(MapEvent event) {
     if (event.source == MapEventSource.mapController && event is MapEventMove) {
-      _interactiveViewerState.interruptAnimatedMovement(event);
+      _interactiveViewerState?.interruptAnimatedMovement(event);
     }
 
     options.onMapEvent?.call(event);

test/map/unlaid_out_map_test.dart

@@ -0,0 +1,113 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:latlong2/latlong.dart';
+
+/// Builds [map] on a route that an opaque route covers from the very first
+/// frame.
+///
+/// The `Overlay` builds the covered route but never lays it out, so the
+/// `LayoutBuilder` inside `FlutterMap` never runs and the map's
+/// `MapInteractiveViewer` is never created - while `onMapReady`, scheduled
+/// from `FlutterMap`'s `initState`, still fires after the first frame.
+///
+/// This happens whenever an app is launched straight into a deeper screen,
+/// for example from a deep link or a notification, with the map underneath.
+Widget _coveredMap(Widget map) => MaterialApp(
+      // Without `onGenerateRoute`, `MaterialApp` builds no `Navigator` and
+      // ignores `onGenerateInitialRoutes`.
+      onGenerateRoute: (_) => null,
+      onGenerateInitialRoutes: (_) => [
+        MaterialPageRoute<void>(builder: (_) => Scaffold(body: map)),
+        MaterialPageRoute<void>(
+          builder: (_) => const Scaffold(body: Text('cover')),
+        ),
+      ],
+    );
+
+Future<void> _uncover(WidgetTester tester) async {
+  tester.state<NavigatorState>(find.byType(Navigator)).pop();
+  await tester.pumpAndSettle();
+}
+
+void main() {
+  testWidgets(
+    'moving the camera from onMapReady does not throw when the map has not '
+    'been laid out, and the move is kept',
+    (tester) async {
+      final controller = MapController();
+      addTearDown(controller.dispose);
+      const destination = LatLng(52, 9);
+      var ready = false;
+
+      await tester.pumpWidget(
+        _coveredMap(
+          FlutterMap(
+            mapController: controller,
+            options: MapOptions(
+              onMapReady: () {
+                ready = true;
+                controller.move(destination, 15);
+              },
+            ),
+            children: const [],
+          ),
+        ),
+      );
+      await tester.pump();
+
+      expect(ready, isTrue);
+      expect(tester.takeException(), isNull);
+
+      await _uncover(tester);
+
+      expect(tester.takeException(), isNull);
+      expect(controller.camera.center, destination);
+      expect(controller.camera.zoom, 15);
+    },
+  );
+
+  testWidgets(
+    'changing the interaction options before the map has been laid out does '
+    'not throw, and the new options apply once it is',
+    (tester) async {
+      final controller = MapController();
+      addTearDown(controller.dispose);
+      final interactive = ValueNotifier(false);
+      addTearDown(interactive.dispose);
+
+      await tester.pumpWidget(
+        _coveredMap(
+          ValueListenableBuilder(
+            valueListenable: interactive,
+            builder: (_, isInteractive, __) => FlutterMap(
+              mapController: controller,
+              options: MapOptions(
+                initialCenter: const LatLng(0, 0),
+                interactionOptions: InteractionOptions(
+                  flags: isInteractive
+                      ? InteractiveFlag.all
+                      : InteractiveFlag.none,
+                ),
+              ),
+              children: const [],
+            ),
+          ),
+        ),
+      );
+      await tester.pump();
+
+      interactive.value = true;
+      await tester.pump();
+
+      expect(tester.takeException(), isNull);
+
+      await _uncover(tester);
+      await tester.drag(find.byType(FlutterMap), const Offset(-200, 0));
+      await tester.pumpAndSettle();
+
+      expect(tester.takeException(), isNull);
+      expect(controller.camera.center, isNot(const LatLng(0, 0)));
+    },
+  );
+}