iOS App NSInvalidArgumentException - objective-c

Question

I have been trying to work through a bug for many hours in my iOS App involving rangeOfString and have finally been able to track down where my app crashes. My app sets TextViewA to the first half of my MainTextView. What if statement should be added should be added to check for nil?
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSCFString rangeOfString:options:range:locale:]: nil argument'
*** First throw call stack:
.
[[myViewController tvTextFirstHalf] setText:[[mainTextView.text componentsSeparatedByString:[[myViewController tvTextMiddle]text]] objectAtIndex:0]];
..............................................................................................
No memory available to program now: unsafe to call malloc
2013-01-04 14:30:58.138 Type[7279:c07] separator: (null)
2013-01-04 14:30:58.138 Type[7279:c07] mainTextView: ; layer = ; contentOffset: {0, 0}
>
2013-01-04 14:30:58.139 Type[7279:c07] mainTextView.text: TESTTTTTTTT f f.
2013-01-04 14:30:58.142 Type[7279:c07] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[__NSCFString rangeOfString:options:range:locale:]: nil argument'
* First throw call stack:
(0x1a5d012 0x1882e7e 0x1a5cdeb 0xf78688 0xf87d88 0xfd05df 0x165ec 0x15dc6 0x1fd7b 0x56d8d5 0x56db3d 0xf74e83 0x1a1c376 0x1a1be06 0x1a03a82 0x1a02f44 0x1a02e1b 0x221d7e3 0x221d668 0x4be65c 0x2c5d 0x2b85)
libc++abi.dylib: terminate called throwing an exception
(gdb)

Answer 1

This oneliners are not considered a good practice in obj-C (i know, it's arguable).
Try to break:
[[myViewController tvTextFirstHalf] setText:[[mainTextView.text componentsSeparatedByString:[[myViewController tvTextMiddle]text]] objectAtIndex:0]];
Into some more debugging-friendly code:
NSString *separator = [[myViewController tvTextMiddle]text];
NSLog (#"separator: %#", separator);
NSLog (#"mainTextView: %#",mainTextView); //is this one nil?
NSLog (#"myViewController.tvTextMiddle: %#", myViewController.tvTextMiddle); //what about this one??
NSLog (#"mainTextView.text: %#",mainTextView.text);
NSArray *components = [mainTextView.text componentsSeparatedByString:separator];
NSLog (#"we found %d components", components.count);
NSString *result = (NSString *)[components objectAtIndex:0];
NSLog (#"result should be: %#", result);
[[myViewController tvTextFirstHalf] setText:result];
The code will break ofcourse but at least you'll know where the problem lies.
You could change last four lines then in something like:
if (components.count >= 1)
{
NSString *result = (NSString *)[components objectAtIndex:0];
NSLog (#"result should be: %#", result);
[[myViewController tvTextFirstHalf] setText:result];
}
else
{
[[myViewController tvTextFirstHalf] setText:#"empty!"];
}
EDIT:
From the NSLog you posted it is obvious that your separator [[myViewController tvTextMiddle]text] is null. Note that null is not the same as empty string. It is unallocated string.
My guess is that actually your tvTextMiddle is null (or if you are using XIB files it is not properly connected to the IBOutlet).
You can check that by adding (somewhere before the breaking-line):
NSLog (#"myViewController.tvTextMiddle: %#", myViewController.tvTextMiddle);

Answer 2

The crash is due to this call componentsSeparatedByString.
If your textview is nil or no text then crash'll occur.
So add a condition like:
if(mainTextView.text.length != 0 && [myViewController tvTextMiddle]text].length != 0)
{
[[myViewController tvTextFirstHalf] setText:[[mainTextView.text componentsSeparatedByString:[[myViewController tvTextMiddle]text]] objectAtIndex:0]];
}