setObjectForKey: object cannot be nil in mutableArray - objective-c

Question

Greeting. I have code like below, I want to add multiple value to the same key in the dictionary.
if ([_sectionContents objectForKey:AddKey] != nil) {
//Already exist a value for the key
id object = [_sectionContents objectForKey:AddKey];
NSMutableArray *objectArray;
if ([object isKindOfClass:[NSMutableArray class]]) {
objectArray = (NSMutableArray *)object;
} else {
NSMutableArray *objectArray = [[NSMutableArray alloc] init];
}
[objectArray addObject:course];
[_sectionContents setObject:objectArray forKey:AddKey];
} else {
//No value for the key
[_sectionContents setObject:[NSArray arrayWithObjects:course,nil] forKey:AddKey];
}
However, I got the error
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** setObjectForKey: object cannot be nil
I figure out it is because of line
[_sectionContents setObject:objectArray forKey:AddKey];
And also I got the warning like objectArray is not used
I don't understand it because I did use objectArray for adding element in it. Why is that please?

Answer 1

When you write
NSMutableArray *objectArray = [[NSMutableArray alloc] init];
you're creating a new variable local to the inside of that 'else' statement. As such, it is created then immediately released, because there is nothing following it in the else statement.
You want to update the variable rather than re-create it - try replacing the rogue line with
objectArray = [[NSMutableArray alloc] init];
(When we don't specify the type e.g. 'NSMutableArray *', we assign to an existing variable, rather than establishing a new one.)

Answer 2

This is because you have two objectArray instead of one
if ([_sectionContents objectForKey:AddKey] != nil) {
...
// first one
NSMutableArray *objectArray;
if ([object isKindOfClass:[NSMutableArray class]]) {
objectArray = (NSMutableArray *)object;
} else {
// second one (*)
NSMutableArray *objectArray = [[NSMutableArray alloc] init];
// *** Correct Solution ***
objectArray = [[NSMutableArray alloc] init];
}
...
} else {
...
}
(*) You've redefined your object array incorrectly in your else clause of your if inside the main if ([_sectionContents objectForKey:AddKey] != nil) {. If object isn't kinda NSMutableArray you get to the else which initialize new ObjectArray and not the one defined at the top of your if statement.