In sequencer, check that selection is not None before merge/create

[user]

In sequencer, I right-clicked a pattern and said "Create pattern from selection". I hadn't selected anything. This error on the terminal:

Traceback (most recent call last):
  File "/usr/local/bin/../share/aldrin/sequencer.py", line 504, in on_popup_create_pattern
    start = (min(self.selection_start[0], self.selection_end[0]), 
TypeError: 'NoneType' object is unsubscriptable

Same with "Merge selected patterns":

Traceback (most recent call last):
  File "/usr/local/bin/../share/aldrin/sequencer.py", line 524, in on_popup_merge
    start = (min(self.selection_start[0], self.selection_end[0]), 
TypeError: 'NoneType' object is unsubscriptable

Attached patch: 1. greys out the menu items if there is no selection and 2. does a try/catch in the merge/create functions to avoid the errors, just in case they're ever called from anywhere else.