Xpra: Ticket #1718: Traceback when running "Start Command" from tray

Server and client are both trunk r17604 Fedora 26 machines with Xpra built from source.

Simple one to reproduce - connect to a session and try to run a new command via the system tray. (with start-new-commands=yes set on the server)

When you click the button, the run command window does not pop up and this traceback is printed in the client output:

Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/xpra/client/gtk_base/gtk_client_base.py", line 278, in show_start_new_command
    self.start_new_command = getStartNewCommand(run_command_cb, self.server_supports_sharing and self.server_supports_window_filters)
AttributeError: 'XpraClient' object has no attribute 'server_supports_sharing'

Of note: the start-child control command works fine.