Why does my app crash

I made a class. This is the h file.

//  MyClass.h
#import <Foundation/Foundation.h>


@interface MyClass : NSObject <NSCoding> {
    NSString *string1;
    NSString *string2;

}
@property (nonatomic, retain) NSString *string1;
@property (nonatomic, retain) NSString *string2;

@end

This is the m file

//  MyClass.m
#import "MyClass.h"


@implementation MyClass
@synthesize string1, string2;

- (void)encodeWithCoder:(NSCoder *)coder;
{
    NSLog(@"encodeWithCoder");

        [coder encodeObject:string1 forKey:@"string1"];
        [coder encodeObject:string2 forKey:@"string2"]; 


}

- (id)initWithCoder:(NSCoder *)coder;
{
    NSLog(@"initWithCoder");
    self = [super init];

        self.string1 = [coder decodeObjectForKey:@"string1"];
        self.string2 = [coder decodeObjectForKey:@"string2"];


    return self;
}


- (void)dealloc {

    [string1 release];
    [string2 release];
    [super dealloc];
}


@end

I created an array of these objects like this

MyClass *object1 = [[MyClass alloc] init];
object1.string1 = @"object1 string1";
object1.string2 = @"string1 string2";
MyClass *object2 = [[MyClass alloc] init];
object2.string1 = @"object2 string1";
object2.string2 = @"object2 string2";
theArray = [[NSMutableArray alloc] initWithObjects:object1, object2, nil];

Then I saved the array like this

[[NSUserDefaults standardUserDefaults] setObject:[NSKeyedArchiver archivedDataWithRootObject:theArray] forKey:@"savedArray"];

Then I loaded the array from disk like this.

NSData *theData = [[NSUserDefaults standardUserDefaults] objectForKey:@"savedArray"];
if (theData != nil) {
    NSLog(@"found something");
    theArray = [NSMutableArray arrayWithArray:[NSKeyedUnarchiver unarchiveObjectWithData:theData]];
}

The program runs normally without error until it gets to this

DetailViewController *detailViewController = [[DetailViewController alloc] initWithNibName:@"DetailViewController" bundle:nil];
     // ...
     // Pass the selected object to the new view controller.
     detailViewController.myObject = [theArray objectAtIndex:indexPath.row];

It crashes on that last line. It only crashes if I load the array using NSUserDefaults, however I don't notice anything I did wrong with that part.

When it crashes this is what the debugger says

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFString objectAtIndex:]: unrecognized selector sent to instance 0x5955e50'
*** Call stack at first throw:
(
    0   CoreFoundation                      0x02395919 __exceptionPreprocess + 185
    1   libobjc.A.dylib                     0x024e35de objc_exception_throw + 47
    2   CoreFoundation                      0x0239742b -[NSObject(NSObject) doesNotRecognizeSelector:] + 187
    3   CoreFoundation                      0x02307116 ___forwarding___ + 966
    4   CoreFoundation                      0x02306cd2 _CF_forwarding_prep_0 + 50
    5   custom object array save test       0x00002872 -[RootViewController tableView:didSelectRowAtIndexPath:] + 156
    6   UIKit                               0x0032b718 -[UITableView _selectRowAtIndexPath:animated:scrollPosition:notifyDelegate:] + 1140
    7   UIKit                               0x00321ffe -[UITableView _userSelectRowAtIndexPath:] + 219
    8   Foundation                          0x00038cea __NSFireDelayedPerform + 441
    9   CoreFoundation                      0x02376d43 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 19
    10  CoreFoundation                      0x02378384 __CFRunLoopDoTimer + 1364
    11  CoreFoundation                      0x022d4d09 __CFRunLoopRun + 1817
    12  CoreFoundation                      0x022d4280 CFRunLoopRunSpecific + 208
    13  CoreFoundation                      0x022d41a1 CFRunLoopRunInMode + 97
    14  GraphicsServices                    0x02bfa2c8 GSEventRunModal + 217
    15  GraphicsServices                    0x02bfa38d GSEventRun + 115
    16  UIKit                               0x002c7b58 UIApplicationMain + 1160
    17  custom object array save test       0x00002160 main + 102
    18  custom object array save test       0x000020f1 start + 53
    19  ???                                 0x00000001 0x0 + 1
)
terminate called after throwing an instance of 'NSException'

Edit:

I was able to fix the problem by replacing this line

theArray = [NSMutableArray arrayWithArray:[NSKeyedUnarchiver unarchiveObjectWithData:theData]];

with this liine

theArray = [[NSMutableArray alloc] initWithArray:[NSKeyedUnarchiver unarchiveObjectWithData:theData]];

so I guess the question now is why did that solve the problem?

I don't see anything wrong with your archiving/unarchiving code. I suspect that indexPath.row is out of range.

EDIT after OP edited question:

The error message says: -[NSCFString objectAtIndex:]: unrecognized selector sent to instance 0x5955e50

You'll notice that this means that 'theArray' is actually pointing at an NSString. Somewhere you have assigned the wrong thing to your 'theArray' variable.

EDIT again after another OP edit:

In your original code you had been using alloc/initWithArray: instead of arrayWithArray:.

The arrayWithArray: usage didn't work for you because that method returns an autoreleased object. So by the time you used it later that reference could have been pointing at just about anything, and hence in one case it happened to be pointing at a string.