ios/iphone - [NSNull dataUsingEncoding:]: unrecognized selector sent to instance - ios

Question

Can someone please help me what could the stated error be for?
[NSNull dataUsingEncoding:]: unrecognized selector sent to instance
I am stuck at printer plugin for an iOS application.
I have been struggling to get this app work for sometime now....the watched code may help find the error...
import "APPPrinter.h"
#interface APPPrinter (Private)
- (UIPrintInteractionController*) getPrintController;
- (UIPrintInteractionController*) adjustSettingsForPrintController:(UIPrintInteractionController*)controller;
- (void) loadContent:(NSString*)content intoPrintController:(UIPrintInteractionController*)controller;
- (void) informAboutResult:(int)code callbackId:(NSString*)callbackId;
- (BOOL) isPrintServiceAvailable;
#end
#implementation APPPrinter
// Is printing available.
- (void) isServiceAvailable:(CDVInvokedUrlCommand*)command
{
CDVPluginResult* pluginResult;
pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK
messageAsBool:[self isPrintServiceAvailable]];
[self.commandDelegate sendPluginResult:pluginResult callbackId:command.callbackId];
}
- (void) print:(CDVInvokedUrlCommand*)command
{
if (![self isPrintServiceAvailable])
{
return;
}
NSArray* arguments = [command arguments];
NSString* content = [arguments objectAtIndex:0];
UIPrintInteractionController* controller = [self getPrintController];
[self adjustSettingsForPrintController:controller];
[self loadContent:content intoPrintController:controller];
[self openPrintController:controller];
[self commandDelegate];
}
- (UIPrintInteractionController*) getPrintController
{
return [UIPrintInteractionController sharedPrintController];
}
- (UIPrintInteractionController*) adjustSettingsForPrintController:(UIPrintInteractionController*)controller
{
UIPrintInfo* printInfo = [UIPrintInfo printInfo];
printInfo.outputType = UIPrintInfoOutputGeneral;
controller.printInfo = printInfo;
controller.showsPageRange = YES;
return controller;
}
- (void) loadContent:(NSString*)content intoPrintController:(UIPrintInteractionController*)controller
{
// Set the base URL to be the www directory.
NSURL *basEURL = [[NSBundle mainBundle] URLForResource:#"Login" withExtension:#"html"];
// Load page into a webview and use its formatter to print the page
UIWebView* webPage = [[UIWebView alloc] init];
[webPage loadHTMLString:content baseURL:basEURL];
UIViewPrintFormatter* formatter = [webPage viewPrintFormatter];
controller.printFormatter = formatter;
controller.showsPageRange = YES;
}
- (void) openPrintController:(UIPrintInteractionController*)controller
{
[self.commandDelegate runInBackground:^
{
[controller presentAnimated:YES completionHandler:NULL];
}];
[self performSelectorOnMainThread:#selector(openPrintController:) withObject:nil waitUntilDone:NO];
}
- (BOOL) isPrintServiceAvailable
{
Class printController = NSClassFromString(#"UIPrintInteractionController");
if (printController)
{
UIPrintInteractionController* controller = [UIPrintInteractionController sharedPrintController];
return (controller != nil) && [UIPrintInteractionController isPrintingAvailable];
}
return NO;
}
#end
Any help/links on creating and getting Print Plugin to run in my app will also be great.

Answer 1

It means that a message "dataUsingEncoding:" was sent to an instance of NSNull.
Somewhere in your program is an object where you apparently think it is an NSString*, but in reality it is an NSNull*. NSNull objects are used as placeholders in arrays and dictionaries to say "there is no value", because you can't store nil in arrays and dictionaries.
In Xcode, click on "Breakpoints", click on the "+", and add an "Exception breakpoint". Run your program again. This time it will stop in the debugger exactly on the code that tries to call dataUsingEncoding: Find out where the object comes from and fix it.