Fix mypy error by asserting
Since we just asked `is_panel_active`, the following `find_output_panel`
*must* succeed.  So we `assert panel` to tell it mypy.
@@ -16,6 +16,7 @@ class SublimeLinterDisplayPanelCommand(sublime_plugin.WindowCommand):
if is_panel_active(window):
panel = window.find_output_panel(PANEL_NAME)
+ assert panel
else:
panel = window.create_output_panel(PANEL_NAME)
syntax_path = "Packages/SublimeLinter/panel/message_view.sublime-syntax"
