Get rid of undefined index warning.

As suggested in issue #2 (closed)

Ever since #2 (closed) was created we are using that patch to suppress the warning successfully. Due to the quick response with !3 (merged) I submit this as PR as well, hoping that we need not to patch the module manually any more.

But as I debugged that function, I did not find any situation where the field group_option is set - Former commit messages suggest that this a preparation for a core feature that is still not implemented. So I wonder whether the lines 56-58 are necessary at all.

As I'm a civcrm newbee I cannot come to a decision at this point.

Closes #2 (closed)