Prev/Next Channel gives traceback when encountering workspace

Workflow to reproduce this (either in Ginga 2.6.5 or dev):

1. Start Ginga.
2. Create a workspace.
3. Press left or right arrow to cycle between channels. The workspace would still be in focus but I see the following traceback on the terminal.

Error making callback 'activated': 'ws1'
Traceback:
  File ".../ginga/misc/Callback.py", line 123, in make_callback
    res = method(*cb_args, **cb_kwdargs)

  File ".../ginga/rv/plugins/Toolbar.py", line 79, in <lambda>
    lambda w: self.fv.next_channel()),

  File ".../ginga/rv/Control.py", line 793, in next_channel
    self.next_channel_ws(ws)

  File ".../ginga/rv/Control.py", line 785, in next_channel_ws
    self.change_channel(chname, raisew=True)

  File ".../ginga/rv/Control.py", line 958, in change_channel
    channel = self.get_channel(name)

  File ".../ginga/rv/Control.py", line 1003, in get_channel
    return self.channel[name]

[ejeschke]

I’ll look at adding something to disable the buttons if there are no images to cycle.

Sorry, please wait a little until I add a couple more commits!

[ejeschke]

Then I believe it the arrows in each workspace toolbar will not surprise anyone with their actions.

Have to qualify this somewhat: when a workspace is added as a subworkspace, and the upper workspace is shown in “grid” mode, there appears to be a bug in how that is displayed. I’ll raise this as a separate issue.