NSArrayM setObject:atIndex:]: object cannot be nil - ios

Question

Having a little issue with a bit of code, returning the error listed above after running its respective action:
- (IBAction)uploadData:(id)sender {
NSMutableArray *uploadArray = [[NSMutableArray alloc]init];
[uploadArray addObject:#"hire.csv"];
[uploadArray addObject:#"equipment.csv"];
[uploadArray addObject:#"signature.png"];
NSString * docPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)objectAtIndex:0];
NSString * proof1 = [docPath stringByAppendingPathComponent:#"proof1.png"];
if([[NSFileManager defaultManager] fileExistsAtPath: proof1])
{
[uploadArray addObject:#"proof1.png"];
}
NSString * proof2 = [docPath stringByAppendingPathComponent:#"proof2.png"];
if([[NSFileManager defaultManager] fileExistsAtPath: proof2])
{
[uploadArray addObject:#"proof2.png"];
}
NSString * proof3 = [docPath stringByAppendingPathComponent:#"proof3.png"];
if([[NSFileManager defaultManager] fileExistsAtPath: proof3])
{
[uploadArray addObject:#"proof3.png"];
}
int arrayCount = [uploadArray count];
int i = 0;
NSString *arrayString;
for (i = 0; i < arrayCount; i++)
{
uploadArray[i] = arrayString;
if(arrayString != nil)
{
NSString * uploadString = [docPath stringByAppendingPathComponent:arrayString];
[self upload:uploadString ftpUrl:#"ftp.xxxxxxxx.com/xx/xx" ftpUsr:#"xxx" ftpPass:#"xxx"];
}
NSLog(uploadArray[i]);
}
}
Now, if I disable everything in the for loop except for the NSLog() I get no error, and it returns the correct list of objects in the array.
This to me suggests that none of the objects should return nil, surely?
Could anyone help me spot what I'm doing wrong? (probably quite a lot as im pretty new to this stuff)

Answer 1

As your error clearly stats, you are adding a nil object to an array and this is not allowed.
In the below code you have not assigned anything to arrayString making it nil. Then you try to add it to your array.
NSString *arrayString;
for (i = 0; i < arrayCount; i++)
{
uploadArray[i] = arrayString;
From the looks of your code you are trying to add every element form your array to your string. You are doing this the wrong way around. You need a NSMutableString and do something like:
[arrayString appendString: uploadArray[i]];