ios - What is the right way to check the Null String in Database in Objective C -

i want restriction on database when in database no data in row can't show exception want restriction on null value

  if ([[utility checknullvalues:tinfo.product_price]isequaltostring:@""]) {         [utility showalertview:@"sorry" message:@"no product buy." viewcontroller:self];       } 

and error:

2015-06-03 12:35:38.320 nicoli[28363:2438247] * terminating app due uncaught exception 'nsinvalidargumentexception', reason: '* +[nsstring stringwithutf8string:]: null cstring' *** first throw call stack:

nsstring *str = ((char *)sqlite3_column_text(compiledstatement, 1)) ? [nsstring stringwithutf8string:(char *)sqlite3_column_text(compiledstatement, 1)] : nil;