I clicked on the 'open' tab which is in the leftmost vertical area. I got a popup:
Slick-C Error
Invalid object handle
file=tbdockchannel.ex offset=452
stack:
Invalid object handle
tbdockchannel.ex 452 _tbDockChanMouseInCallback(1,_tbopen_form,10038,Open,1,1) p_window_id: 44 p_object: OI_FORM p_name: _dock_channel_form
dockchannel.ex 4038 dockchannel:_dcCtlOnMouseIn(_tbopen_form,1,0) p_window_id: 45 p_object: OI_IMAGE p_name:
dockchannel.ex 3853 dockchanSelect(1:_tbopen_form) p_window_id: 45 p_object: OI_IMAGE p_name:
dockchannel.ex 3899 _dock_channel_form.lbutton_up() p_window_id: 45 p_object: OI_IMAGE p_name:
I quit SE and re-launched, and was able to open the intended file.