unit tests for setStackRoot

lib/ios/RNNNavigationStackManager.m

 -(void)setRoot:(UIViewController<RNNRootViewProtocol> *)newRoot fromComponent:(NSString *)componentId completion:(RNNTransitionCompletionBlock)completion {
 	UIViewController* vc = [_store findComponentForId:componentId];
 	UINavigationController* nvc = [vc navigationController];
+	
+	if (!nvc) {
+		@throw [NSException exceptionWithName:@"StackNotFound" reason:@"Missing stack, setRoot should contain stack layout" userInfo:nil];
+	}
+	
 	[CATransaction begin];
 	[CATransaction setCompletionBlock:^{
 		if (completion) {

lib/ios/ReactNativeNavigationTests/RNNNavigationStackManagerTest.m

 
 }
 
+- (void)testStackRoot_navigationControllerNilShouldThrow {
+	[self.nvc setViewControllers:@[self.vc1]];
+	self.nvc.willReturnVCs = @[self.vc1];
+	
+	XCTAssertThrowsSpecificNamed([self.uut setRoot:self.vc2 fromComponent:nil completion:nil], NSException, @"StackNotFound");
+}
 
-
+- (void)testStackRoot_shouldUpdateNavigationControllerChildrenViewControllers {
+	self.nvc.willReturnVCs = @[self.vc1, self.vc2, self.vc3];
+	
+	[self.uut setRoot:self.vc2 fromComponent:@"vc1" completion:nil];
+	
+	XCTAssertEqual(self.nvc.viewControllers.lastObject, self.vc2);
+	XCTAssertEqual(self.nvc.viewControllers.count, 1);
+}
 
 @end