Reject pop command when viewController not found in the hierarchy (#6048)

Until now the promise was rejected only on Android.

lib/ios/RNNCommandsHandler.m

 	RNNAssertMainQueue();
     
 	RNNComponentViewController *vc = (RNNComponentViewController*)[RNNLayoutManager findComponentForId:componentId];
-	RNNNavigationOptions *options = [[RNNNavigationOptions alloc] initWithDict:mergeOptions];
-	[vc overrideOptions:options];
-	
-	[vc.stack pop:vc animated:[vc.resolveOptionsWithDefault.animations.pop.enable getWithDefaultValue:YES] completion:^{
-		[_eventEmitter sendOnNavigationCommandCompletion:pop commandId:commandId params:@{@"componentId": componentId}];
-		completion();
-	} rejection:rejection];
+    if (vc) {
+        RNNNavigationOptions *options = [[RNNNavigationOptions alloc] initWithDict:mergeOptions];
+        [vc overrideOptions:options];
+        
+        [vc.stack pop:vc animated:[vc.resolveOptionsWithDefault.animations.pop.enable getWithDefaultValue:YES] completion:^{
+            [self->_eventEmitter sendOnNavigationCommandCompletion:pop commandId:commandId params:@{@"componentId": componentId}];
+            completion();
+        } rejection:rejection];
+    } else {
+        [RNNErrorHandler reject:rejection withErrorCode:1012 errorDescription:[NSString stringWithFormat:@"Popping component failed - componentId '%@' not found", componentId]];
+    }
 }
 
 - (void)popTo:(NSString*)componentId commandId:(NSString*)commandId mergeOptions:(NSDictionary *)mergeOptions completion:(RNNTransitionCompletionBlock)completion rejection:(RCTPromiseRejectBlock)rejection {

playground/ios/NavigationTests/RNNCommandsHandlerTest.m

 	[self.modalManager verify];
 }
 
+- (void)testPop_shouldRejectPromiseForInvalidComponentId {
+	[self.uut setReadyToReceiveCommands:true];
+	
+    XCTestExpectation *expectation = [self expectationWithDescription:@"Should invoke reject block"];
+
+	[self.uut pop:@"invalidComponentId" commandId:@"pop" mergeOptions:nil completion:^{
+		[expectation fulfill];
+	} rejection:^(NSString *code, NSString *message, NSError *error) {
+		XCTAssert([code isEqualToString:@"1012"]);
+		XCTAssert([message isEqualToString:@"Popping component failed - componentId 'invalidComponentId' not found"]);
+		[expectation fulfill];
+	}];
+	
+	[self waitForExpectationsWithTimeout:5 handler:nil];
+}
 
 @end