Allow for nil placeholder text

[jsm174]

I have an app with a textview that can go into a "Read Only" mode. When in "Read Only" mode the placeholder gets set to nil, even if the field value is empty or nil.

notesTextView.placeholder = (notesTextView.editable) ?
    @"Enter additional notes..." : nil;

This used to work in SSTextView, however in SAMTextView it crashes:

[89217:450317] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'NSConcreteAttributedString initWithString:: nil value'
.
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4   Foundation                          0x0000000103a1eafd -[NSConcreteAttributedString initWithString:attributes:] + 27
5   App                      0x000000010290a664 -[SAMTextView setPlaceholder:] + 1060