This method takes two arguments although one is obviously a place holder. Both arguments are mandatory. The first indicates the dialog to be displayed while the second is a null value.
The values for selecting browser UI dialogs are:
The API for this method breaks a rule of requiring a place holder argument to be set to null and yet mandating that it must be present. This is poor API design. It is possible that the argument may take undocumented values other than null, which may alter its behavior but it should still be optional.