Error in iOS: Mutating method sent to immutable object

Today I switch account login, access to user information, the program crashes, error: Mutating method sent to immutable object

When I set a global breakpoint to debug, this is to give us the method of error, and print the error message

This is where we call the method, where self.msgdict is the immutable dictionary and changes it to a mutable dictionary.

In addition: Mutating method sent to immutable object means that you want to invoke a variable object by using an immutable object called

Error in iOS: Mutating method sent to immutable object