Navigation controller push nil view controller error

Question

I'm having some problems with a Navigation controller in my app. Moving from the RootViewController to a UITableViewController works fine. I now want to have one more level of drilldown, so users pick an item from the list and a new screen appears, like this:

RootViewController --> TableViewController --> ItemViewController

I've used the exact same code that switches the first views, but I get:

Application tried to push a nil view controller on target <UINavigationController...

The code is identical to the first one, so how can it be nil?

In RootViewController.h:

@interface RootViewController : UIViewController {
    IBOutlet TableViewController *tableViewController;
}

@property (nonatomic, retain) TableViewController * tableViewController;

In the .m file I synthesize the property and then use a button to call:

[self.navigationController pushViewController:tableViewController animated:YES];

In TableViewController.h:

#import "ItemDetailViewController.h"

@class TableViewController;

@interface TableViewController : UITableViewController {
    IBOutlet ItemDetailViewController * itemDetailViewController;
}

@property (nonatomic, retain) ItemDetailViewController * itemDetailViewController;

@end

And again, I synthesize it in the .m file and use code to push the new view in didSelectRowAtIndexPath:

[self.navigationController pushViewController:itemDetailViewController animated:YES];

When you tap on an item I get the error message above. Does anyone have any idea why this is happening?

What is the connection between 'itemDetailViewController' and 'plantDetailViewController'? — Tejas
– Tejas, Commented Jun 2, 2011 at 16:33
3 nibs, one for each view. RootViewController, TableViewController, ItemViewController. — ThisDarkTao
– ThisDarkTao, Commented Jun 2, 2011 at 16:50

Rob Napier · Accepted Answer · 2011-06-02 16:35:57Z

1

This most common cause of this error is failing to connect itemDetailViewController to the actual object in IB. Note that you should generally place the IBOutlet designator on the property rather than on the ivar.

answered Jun 2, 2011 at 16:35

Rob Napier

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7 Comments

ThisDarkTao Over a year ago

Everything is connected up as per the RootViewController, and all connections are there and correct. I will however switch the IBOutlet designators, maybe its that?

Rob Napier Over a year ago

In the debugger, is itemDetailViewController nil? Is it nil in TableViewController viewDidLoad? If so, then you're not connected to what you think you're connected to.

ThisDarkTao Over a year ago

How can I check that? Do you mean use instruments and check for memory allocation?

ThisDarkTao Over a year ago

Okay just did an if statement to check if self.itemDetailViewController is nil, and it is. Any idea why? Everything is definitely connected up the same as the first view.

Rob Napier Over a year ago

You can also set a breakpoint and po itemDetailViewController. A common reason is that your NIB has not loaded yet (viewDidLoad has not run). This doesn't happen until the view is first accessed. Another common mistake is having duplicate objects. This commonly happens when people wire an object in the NIB and also create an object of the same class using +alloc. The two objects are unrelated. Another possibility is that you're not loading the NIB you think you are. Put breakpoints in -viewDidLoad and see when things happen.

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