[RootModel setAppLevel:]: unrecognized selector sent to instance

I have a class called RootModel. In RootModel.h I have:

@interface RootModel : NSObject {
    NSString *appLevel;
}
@property(nonatomic, retain) NSString *appLevel;

In RootModel.m I have the following:

#import "RootModel.h"
#import "MainViewController.h"
@implementation RootModel
@synthesize   appLevel;
#pragma mark Singleton Methods

static RootModel *sharedObject = nil;

+(id)sharedModel {
    @synchronized(self){
        if(sharedObject == nil)
            sharedObject = [[super allocWithZone:NULL] 开发者_JS百科init];

    }
    return sharedObject;
}

in MainViewController.m I have:
RootModel *rm = [RootModel sharedModel];
rm.appLevel = @"0";

The last statement give me [RootModel setAppLevel:]: unrecognized selector sent to instance. It should be noted that everything was working fine with the above code. My project somehow got corrupted and I built a new project using the files and now I get this error. Thanks for any help.

in RootModel.h

@class MainViewController;//note Here

@interface RootModel : NSObject {

           NSString *appLevel;

}

@property(nonatomic, retain) NSString *appLevel;

+(id)sharedModel;//Note Here

@end

You may have forgotten this method

+ (id)allocWithZone:(NSZone *)zone {

    return [[self sharedManager] retain];

}

Take a look at this.