When running code off Master:

1. Add two files of different length
2. Select both files
3. Select the vale of the length tab in the table view:
   
4. Copy with ctrl-c

Crash:

Traceback (most recent call last):
  File "D:\projects\picard\picard\ui\metadatabox_init_.py", line 261, in keyPressEvent
    self._copy_value()
  File "D:\projects\picard\picard\ui\metadatabox_init_.py", line 283, in _copy_value
    value = [format_time(value or 0), ]
             ^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\projects\picard\picard\util_init_.py", line 305, in format_time
    ms = float(ms)
         ^^^^^^^^^
TypeError: float() argument must be a string or a real number, not 'list'

Specifically, when copying the value from ~length with multiple selection, the value is [''], because the tag_diff contains multiple values, so 
self.tag_diff.new['~length'] = ['']