Do not call callback method when not exists

[kination]

When callback method is not defined, it calls error log after import.

Error: scope.callback is not a function

I updated to call the method only when it is not null. Please look on.

[lbicknese]

This change will get rid of the error message, but it might be better to change the binding on the attribute. Right now it is using =; it should use &. This resolves the error, even if callback is undefined, and allows the use of functions more in line with angular conventions.

[kination]

@lbicknese good suggestion. @bahaaldine how do you think?

[bahaaldine]

thanks @djKooks