fix(view-controller): dismiss does not crash when called more than once

fixes #8395

src/navigation/test/view-controller.spec.ts

@@ -117,8 +117,26 @@ describe('ViewController', () => {
 
       viewController.dismiss('didDismiss data');
     }, 10000);
+
+    it('should not crash when calling dismiss() twice', (done) => {
+      // arrange
+      let viewController = mockView();
+      let navControllerBase = mockNavController();
+      mockViews(navControllerBase, [viewController]);
+
+      viewController.onDidDismiss((data: any) => {
+        expect(data).toEqual('didDismiss data');
+        setTimeout(() => {
+          viewController.dismiss(); // it should not crash
+          done();
+        }, 100);
+      });
+
+      viewController.dismiss('didDismiss data');
+    }, 10000);
   });
 
+
   afterEach(() => {
     if (subscription) {
       subscription.unsubscribe();

src/navigation/view-controller.ts

@@ -180,6 +180,10 @@ export class ViewController {
    *
    */
   dismiss(data?: any, role?: any, navOptions: NavOptions = {}) {
+    if (!this._nav) {
+      return Promise.resolve(false);
+    }
+
     let options = merge({}, this._leavingOpts, navOptions);
     this._onWillDismiss && this._onWillDismiss(data, role);
     return this._nav.remove(this._nav.indexOf(this), 1, options).then(() => {