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Is memory leak causing crash here, or class other implementation issue?
#import <Foundation/Foundation.h>
@interface Mutation : NSObject
@property (assign) NSString *inputString;
@property (assign) NSString *outputString;
@end

#import "Mutation.h"
@implementation Mutation
@synthesize inputString;
@synthesize outputString;
@end

#import <Cocoa/Cocoa.h>
#import "Mutation.h"
@interface NTAppDelegate : NSObject <NSApplicationDelegate>
@property (assign) IBOutlet NSWindow *window;
@property (weak) IBOutlet NSTextField *dataField;
@property (weak) IBOutlet NSTextField *outputField;
@property (assign) Mutation *mutation;
- (IBAction)receiveUserTextFromTextField:(NSTextField *)sender;
@end

#import "NTAppDelegate.h"
#import "Mutation.h"
@implementation NTAppDelegate
@synthesize window = _window;
@synthesize dataField = _dataField;
@synthesize outputField = _outputField;
@synthesize mutation = mutation;    //statement #1

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification //#3 (block)
{
Mutation *aMutation = [[Mutation alloc] init];
[self setMutation:aMutation];
[aMutation setInputString:@"new"];
[aMutation setOutputString:@"old"];
NSLog(@"Mutation inputString is %@; outputString is %@", [aMutation inputString], [aMutation outputString]);
}

- (IBAction)receiveUserTextFromTextField:(NSTextField *)sender   //#2 (block)
{
// assign the user's entered text to Mutation's inputString
NSString* newText = [sender stringValue];   // -stringValue inherited from NSControl
NSLog (@"%@ was entered", newText);         //  <-THIS WORKS
[mutation setInputString:newText];           //  <-CRASH  statement #4 (crashes)
NSLog(@"Mutation(2) inputString is %@; outputString is %@", [aMutation inputString], [aMutation outputString]);
}

@end