fix pop gesture problem

ios/Classes/Boost/FlutterBoostPlugin.m

@@ -95,9 +95,19 @@
         pageInfo[@"uniqueId"] = [FlutterBoostPlugin sharedInstance].fPageId;
         if(result) result(pageInfo);
     } else if([@"disablePopGesture" isEqualToString:call.method]) {
-        [FlutterBoostPlugin sharedInstance].application.flutterViewController.navigationController.interactivePopGestureRecognizer.enabled = NO;
+        
+        
+        FLBFlutterViewContainer *flutterViewController = [FlutterBoostPlugin sharedInstance].application.flutterViewController;
+        if ([flutterViewController isKindOfClass:flutterViewController.class]) {
+            flutterViewController.navigationController.interactivePopGestureRecognizer.enabled = NO;
+            flutterViewController.disablePopGesture = @(YES);
+        }
     } else if([@"enablePopGesture" isEqualToString:call.method]){
-        [FlutterBoostPlugin sharedInstance].application.flutterViewController.navigationController.interactivePopGestureRecognizer.enabled = YES;
+        FLBFlutterViewContainer *flutterViewController = [FlutterBoostPlugin sharedInstance].application.flutterViewController;
+        if ([flutterViewController isKindOfClass:flutterViewController.class]) {
+            flutterViewController.navigationController.interactivePopGestureRecognizer.enabled = YES;
+            flutterViewController.disablePopGesture = @(NO);
+        }
     } else{
         result(FlutterMethodNotImplemented);
     }

ios/Classes/container/FLBFlutterViewContainer.h

@@ -29,6 +29,7 @@
 NS_ASSUME_NONNULL_BEGIN
 @interface FLBFlutterViewContainer  : FlutterViewController<FLBFlutterContainer>
 @property (nonatomic,copy,readwrite) NSString *name;
+@property (nonatomic, strong) NSNumber *disablePopGesture;
 - (instancetype)init NS_DESIGNATED_INITIALIZER;
 - (void)surfaceUpdated:(BOOL)appeared;
 @end

ios/Classes/container/FLBFlutterViewContainer.m

@@ -275,6 +275,14 @@ static NSUInteger kInstanceCounter = 0;
     [self surfaceUpdated:YES];
     
     [super viewDidAppear:animated];
+    
+    
+    
+    // Enable or disable pop gesture
+    // note: if disablePopGesture is nil, do nothing
+    if (self.disablePopGesture) {
+        self.navigationController.interactivePopGestureRecognizer.enabled = ![self.disablePopGesture boolValue];
+    }
 }
 
 - (void)viewWillDisappear:(BOOL)animated