hi , every time i press preview this message its shown

"An unhandled exception of type 'System.InvalidOperationException' occurred in BlueFlame 1.0.exe

Additional information: An error occurred creating the form. See Exception.InnerException for details. The error is: Object reference not set to an instance of an object."

i haded to star my project like 2 times for the error ..really need help

i am really mad beacause of that problem