Fix null event crash in TapGestureRecognizer (#45943)

packages/flutter/lib/src/gestures/recognizer.dart

@@ -398,6 +398,9 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
 
   /// If non-null, the recognizer will call [didExceedDeadline] after this
   /// amount of time has elapsed since starting to track the primary pointer.
+  ///
+  /// The [didExceedDeadline] will not be called if the primary pointer is
+  /// accepted, rejected, or all pointers are up or canceled before [deadline].
   final Duration deadline;
 
   /// The maximum distance in logical pixels the gesture is allowed to drift
@@ -495,8 +498,11 @@ abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecogni
 
   @override
   void acceptGesture(int pointer) {
+    if (pointer == primaryPointer) {
+      _stopTimer();
       _gestureAccepted = true;
     }
+  }
 
   @override
   void rejectGesture(int pointer) {

packages/flutter/test/gestures/tap_test.dart

@@ -582,6 +582,51 @@ void main() {
     drag.dispose();
   });
 
+  testGesture('non-primary pointers does not trigger timeout', (GestureTester tester) {
+    // Regression test for https://github.com/flutter/flutter/issues/43310
+    // Pointer1 down, pointer2 down, then pointer 1 up, all within the timeout.
+    // In this way, `BaseTapGestureRecognizer.didExceedDeadline` can be triggered
+    // after its `_reset`.
+    final TapGestureRecognizer tap = TapGestureRecognizer();
+
+    final List<String> recognized = <String>[];
+    tap.onTapDown = (_) {
+      recognized.add('down');
+    };
+    tap.onTapUp = (_) {
+      recognized.add('up');
+    };
+    tap.onTap = () {
+      recognized.add('tap');
+    };
+    tap.onTapCancel = () {
+      recognized.add('cancel');
+    };
+
+    tap.addPointer(down1);
+    tester.closeArena(down1.pointer);
+
+    tap.addPointer(down2);
+    tester.closeArena(down2.pointer);
+
+    expect(recognized, isEmpty);
+
+    tester.route(up1);
+    GestureBinding.instance.gestureArena.sweep(down1.pointer);
+    expect(recognized, <String>['down', 'up', 'tap']);
+    recognized.clear();
+
+    // If regression happens, the following step will throw error
+    tester.async.elapse(const Duration(milliseconds: 200));
+    expect(recognized, isEmpty);
+
+    tester.route(up2);
+    GestureBinding.instance.gestureArena.sweep(down2.pointer);
+    expect(recognized, isEmpty);
+
+    tap.dispose();
+  });
+
   group('Enforce consistent-button restriction:', () {
     // Change buttons during down-up sequence 1
     const PointerMoveEvent move1lr = PointerMoveEvent(