Mysterious UIButton crash - ios

Question

I have a UITableViewCell that has 2 UIButton instances. Both buttons have a default image. Only the "bad" button has an image for the disabled state. Nothing more.
In the cell class these two buttons are declared as properties with the IBOutlet keyword:
#property (nonatomic, readonly) IBOutlet UIButton* buttonCall;
#property (nonatomic, readonly) IBOutlet UIButton* buttonGPS;
The connections between the 2 buttons in the .xib and the properties in the class are made.
So, when the .xib is loaded the following is done:
MyCell* cell = (MyCell*)[tableView dequeueReusableCellWithIdentifier:cellID];
if (nil == cell)
{
NSArray* nibContents = [[NSBundle mainBundle] loadNibNamed:#"MyCell" owner:nil options:nil];
cell = [nibContents objectAtIndex:0];
cell.selectionStyle = UITableViewCellSelectionStyleNone;
}
// gps button
[cell.buttonGPS setTitle:[NSString stringWithFormat:#"%d", indexPath.row] forState:UIControlStateNormal];
[cell.buttonGPS addTarget:self action:#selector(actionGPS:) forControlEvents:UIControlEventTouchUpInside];
if (...condition...)
{
cell.buttonCall.enabled = NO;
[cell.buttonCall removeTarget:self action:#selector(actionCall:) forControlEvents:UIControlEventTouchUpInside];
}
else
{
cell.buttonCall.enabled = YES;
[cell.buttonCall setTitle:[...TheTitleString...] forState:UIControlStateNormal];
[cell.buttonCall addTarget:self action:#selector(actionCall:) forControlEvents:UIControlEventTouchUpInside];
}
When the view controller appears it goes successfully through (UITableViewCell*)tableView:cellForRowAtIndexPath: the necessary number of times. Then the app crashes with the following stack:
0 kill
...
[UIButton imageRectForContentRect:]
[UIButton(UIButtonInternal) _setupImageView]
[UIButton layoutSubviews]
...
start
I removed the image of the button and started crashing with stack:
0 kill
...
[UIButton layoutSubviews]
...
start
I added some identifier labels for the buttons - still crashing.
Removed the XCode-InterfaceBuilder connection for the "bad" button and the crashes stopped. But the button is not functional.
Any idea what is causing the crashes?
Call stack is:
0 CoreFoundation 0x013d406e __exceptionPreprocess + 206
1 libobjc.A.dylib 0x019f0d0a objc_exception_throw + 44
2 CoreFoundation 0x013d5ced -[NSObject doesNotRecognizeSelector:] + 253
3 CoreFoundation 0x0133af00 ___forwarding___ + 432
4 CoreFoundation 0x0133ace2 _CF_forwarding_prep_0 + 50
5 UIKit 0x0086de3a -[UIButton imageRectForContentRect:] + 350
6 UIKit 0x0086ed5b -[UIButton(UIButtonInternal) _setupImageView] + 158
7 UIKit 0x0086e5f8 -[UIButton layoutSubviews] + 693
8 UIKit 0x0069f322 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 178
9 CoreFoundation 0x013d5e72 -[NSObject performSelector:withObject:] + 66
10 QuartzCore 0x0042092d -[CALayer layoutSublayers] + 266
11 QuartzCore 0x0042a827 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 231
12 QuartzCore 0x003b0fa7 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 377
13 QuartzCore 0x003b2ea6 _ZN2CA11Transaction6commitEv + 374
14 QuartzCore 0x003b2580 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 80
15 CoreFoundation 0x013a89ce __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 30
16 CoreFoundation 0x0133f670 __CFRunLoopDoObservers + 384
17 CoreFoundation 0x0130b4f6 __CFRunLoopRun + 1174
18 CoreFoundation 0x0130adb4 CFRunLoopRunSpecific + 212
19 CoreFoundation 0x0130accb CFRunLoopRunInMode + 123
20 GraphicsServices 0x03258879 GSEventRunModal + 207
21 GraphicsServices 0x0325893e GSEventRun + 114
22 UIKit 0x00660a9b UIApplicationMain + 1175
23 MyApp 0x000023f9 main + 169
24 MyApp 0x00002345 start + 53
Exception is:
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull length]: unrecognized selector sent to instance 0x146acd8'
info symbol 0x146acd8
__kCFNull in section LC_SEGMENT.__DATA.__data of /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation
The second scenario is similar. The exception, however, is exactly the same. Here is the call stack:
0 CoreFoundation 0x013d406e __exceptionPreprocess + 206
1 libobjc.A.dylib 0x019f0d0a objc_exception_throw + 44
2 CoreFoundation 0x013d5ced -[NSObject doesNotRecognizeSelector:] + 253
3 CoreFoundation 0x0133af00 ___forwarding___ + 432
4 CoreFoundation 0x0133ace2 _CF_forwarding_prep_0 + 50
5 UIKit 0x0086e851 -[UIButton layoutSubviews] + 1294
6 UIKit 0x0069f322 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 178
7 CoreFoundation 0x013d5e72 -[NSObject performSelector:withObject:] + 66
8 QuartzCore 0x0042092d -[CALayer layoutSublayers] + 266
9 QuartzCore 0x0042a827 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 231
10 QuartzCore 0x003b0fa7 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 377
11 QuartzCore 0x003b2ea6 _ZN2CA11Transaction6commitEv + 374
12 QuartzCore 0x003b2580 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 80
13 CoreFoundation 0x013a89ce __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 30
14 CoreFoundation 0x0133f670 __CFRunLoopDoObservers + 384
15 CoreFoundation 0x0130b4f6 __CFRunLoopRun + 1174
16 CoreFoundation 0x0130adb4 CFRunLoopRunSpecific + 212
17 CoreFoundation 0x0130accb CFRunLoopRunInMode + 123
18 GraphicsServices 0x03258879 GSEventRunModal + 207
19 GraphicsServices 0x0325893e GSEventRun + 114
20 UIKit 0x00660a9b UIApplicationMain + 1175
21 MyApp 0x000023f9 main + 169
22 MyApp 0x00002345 start + 53

Answer 1

From the exception, it looks like you think you are passing an NSString to the title of a button, and you are in fact passing an instance of NSNull. This is quite common if you are deriving your table view data from a JSON source or similar - NSNull is used if the source field does not exist.
The exception you get is that length is not a recognised selector for NSNull, which is true, and it seems to come up when the button is laying out its subviews, which it would do when you assign a new title. So I would focus your investigation in that area. Check to see what you are actually sending for the title of the button.

Answer 2

MyCell* cell = (MyCell*)[tableView dequeueReusableCellWithIdentifier:cellID];
if (nil == cell)
{
NSArray* nibContents = [[NSBundle mainBundle] loadNibNamed:#"MyCell" owner:nil options:nil];
cell = [nibContents objectAtIndex:0];
cell.selectionStyle = UITableViewCellSelectionStyleNone;
[cell.buttonGPS addTarget:self action:#selector(actionGPS:) forControlEvents:UIControlEventTouchUpInside];
[cell.buttonCall addTarget:self action:#selector(actionCall:) forControlEvents:UIControlEventTouchUpInside];
}
// gps button
[cell.buttonGPS setTitle:[NSString stringWithFormat:#"%d", indexPath.row] forState:UIControlStateNormal];
if (...condition...)
{
cell.buttonCall.enabled = NO;
}
else
{
cell.buttonCall.enabled = YES;
[cell.buttonCall setTitle:[...TheTitleString...] forState:UIControlStateNormal];
}
Make these changes in ur code & check. I think some condition may be wrong, so b4 adding the target it was removed and when attempted it was crashing.