Merge pull request #694 from CoderMJLee/fix/NSNull-class-recursive-problem(because-of-MessageUI-in-Xcode-11-beta-1-with-iOS-13-beta-1)

fix a recursive problem that brought in by MessageUI (Xcode 11 & iOS 13 beta1)

Author: kinarobin

MJExtension/MJFoundation.m

@@ -22,15 +22,16 @@ + (BOOL)isClassFromFoundation:(Class)c
     dispatch_once(&onceToken, ^{
         // 集合中没有NSObject，因为几乎所有的类都是继承自NSObject，具体是不是NSObject需要特殊判断
         foundationClasses = [NSSet setWithObjects:
-                              [NSURL class],
-                              [NSDate class],
-                              [NSValue class],
-                              [NSData class],
-                              [NSError class],
-                              [NSArray class],
-                              [NSDictionary class],
-                              [NSString class],
-                              [NSAttributedString class], nil];
+                             [NSNull class],
+                             [NSURL class],
+                             [NSDate class],
+                             [NSValue class],
+                             [NSData class],
+                             [NSError class],
+                             [NSArray class],
+                             [NSDictionary class],
+                             [NSString class],
+                             [NSAttributedString class], nil];
     });
 
     __block BOOL result = NO;