Fix a crash on macOS 10.11.

[antons]

• delegate and dataSource are assign (notweak) on macOS 10.11.
• dataSource was set to nil before delegate, and when dataSource is changed NSTableView notifies the delegate about removing all rows.
• #58 introduced an issue where _delegateDataSourceProxy was released before setting delegate and dataSource to nil.
• When NSTableView attempted to notify the delegate about removing all rows it was already released, which caused crashes.