[codeOfJannik]

Fixes #2293

MapControllerImpl._interactiveViewerState is late and only assigned once the map's MapInteractiveViewer is created, which happens during layout. onMapReady fires after the first frame regardless, so a FlutterMap that is built but not laid out (e.g. on a route covered by another route from the first frame) throws LateInitializationError on any controller move (_emitMapEvent) and on any interactionOptions change (options=). See the issue for the details and a minimal app.

Changes

• _interactiveViewerState is now nullable.
• options= skips updateGestures while there is no viewer. The viewer builds its gestures from the controller's current options in didChangeDependencies, so it starts out with the latest options.
• _emitMapEvent skips interruptAnimatedMovement while there is no viewer. There is no viewer-driven gesture animation to interrupt.

Behaviour for maps that have been laid out is unchanged.

Tests

New test/map/unlaid_out_map_test.dart, with the map on a route that an opaque route covers from the first frame:

• moving the camera from onMapReady does not throw, and the move is kept once the map is shown;
• changing the interaction options before the map is laid out does not throw, and the new options apply once it is (verified by dragging the map).

Both tests fail on master with LateInitializationError: Field '_interactiveViewerState@…' has not been initialized. and pass with this change. The full suite passes (142 tests) and dart analyze is clean.