Fix crash when calling setStackRoot multiple times in quick succession (#4843)

Previous push animation wasn’t cancelled, when it ended the controller tried
to destroy previous view which was already destroyed.

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/stack/StackController.java

@@ -192,6 +192,7 @@ public class StackController extends ParentController<StackLayout> {
     }
 
     public void setRoot(List<ViewController> children, CommandListener listener) {
+        animator.cancelPushAnimations();
         if (children.size() == 1) {
             backButtonHelper.clear(CollectionUtils.last(children));
             push(CollectionUtils.last(children), new CommandListenerAdapter() {

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/stack/StackControllerTest.java

@@ -281,6 +281,19 @@ public class StackControllerTest extends BaseTest {
         });
     }
 
+    @Test
+    public void setRoot_doesNotCrashWhenCalledInQuickSuccession() {
+        disablePushAnimation(child1);
+        uut.setRoot(Collections.singletonList(child1), new CommandListenerAdapter());
+
+        uut.setRoot(Collections.singletonList(child2), new CommandListenerAdapter());
+        uut.setRoot(Collections.singletonList(child3), new CommandListenerAdapter());
+        animator.endPushAnimation(child2.getView());
+        animator.endPushAnimation(child3.getView());
+
+        assertContainsOnlyId(child3.getId());
+    }
+
     @Test
     public synchronized void pop() {
         disablePushAnimation(child1, child2);