[google_maps_flutter_platform_interface] Fixes for custom tiles (#3449)

Author: Chris Yang

packages/google_maps_flutter/google_maps_flutter_platform_interface/lib/src/method_channel/method_channel_google_maps_flutter.dart

@@ -193,9 +193,13 @@ class MethodChannelGoogleMapsFlutter extends GoogleMapsFlutterPlatform {
         final Map<TileOverlayId, TileOverlay> tileOverlaysForThisMap =
             _tileOverlays[mapId];
         final String tileOverlayId = call.arguments['tileOverlayId'];
-        final TileOverlay tileOverlay = tileOverlaysForThisMap[tileOverlayId];
-        assert(tileOverlay.tileProvider.getTile != null);
-        final Tile tile = await tileOverlay.tileProvider.getTile(
+        final TileOverlay tileOverlay =
+            tileOverlaysForThisMap[TileOverlayId(tileOverlayId)];
+        Tile tile;
+        if (tileOverlay == null || tileOverlay.tileProvider == null) {
+          return TileProvider.noTile.toJson();
+        }
+        tile = await tileOverlay.tileProvider.getTile(
           call.arguments['x'],
           call.arguments['y'],
           call.arguments['zoom'],

packages/google_maps_flutter/google_maps_flutter_platform_interface/lib/src/types/types.dart

@@ -29,3 +29,4 @@ export 'utils/circle.dart';
 export 'utils/marker.dart';
 export 'utils/polygon.dart';
 export 'utils/polyline.dart';
+export 'utils/tile_overlay.dart';