gui: Fix crash when quickly opening/closing applets

Quickly closing/reopening applets (e.g. quickly clicking the checkbox
on an entire folder of applets) would previously lead to an occasional
KeyError on the self.dock_to_item access in on_dock_closed, as close()
would be invoked more than once.

The geometry/checked state handling can potentially be cleaned up
further, but at least this avoid the crash.

artiq/gui/applets.py

@@ -321,7 +321,6 @@ def __init__(self, main_window, datasets_sub):
 
         self.main_window = main_window
         self.datasets_sub = datasets_sub
-        self.dock_to_item = dict()
         self.applet_uids = set()
 
         self.table = QtWidgets.QTreeWidget()
@@ -414,12 +413,12 @@ def set_spec(self, item, spec):
         finally:
             self.table.itemChanged.connect(self.item_changed)
 
-    def create(self, uid, name, spec):
-        dock = _AppletDock(self.datasets_sub, uid, name, spec)
+    def create(self, item, name, spec):
+        dock = _AppletDock(self.datasets_sub, item.applet_uid, name, spec)
         self.main_window.addDockWidget(QtCore.Qt.RightDockWidgetArea, dock)
         dock.setFloating(True)
         asyncio.ensure_future(dock.start())
-        dock.sigClosed.connect(partial(self.on_dock_closed, dock))
+        dock.sigClosed.connect(partial(self.on_dock_closed, item, dock))
         return dock
 
     def item_changed(self, item, column):
@@ -437,15 +436,15 @@ def item_changed(self, item, column):
                     if item.applet_dock is None:
                         name = item.text(0)
                         spec = self.get_spec(item)
-                        dock = self.create(item.applet_uid, name, spec)
+                        dock = self.create(item, name, spec)
                         item.applet_dock = dock
                         if item.applet_geometry is not None:
                             dock.restoreGeometry(item.applet_geometry)
                             # geometry is now handled by main window state
                             item.applet_geometry = None
-                        self.dock_to_item[dock] = item
                 else:
                     dock = item.applet_dock
+                    item.applet_dock = None
                     if dock is not None:
                         # This calls self.on_dock_closed
                         dock.close()
@@ -455,12 +454,9 @@ def item_changed(self, item, column):
         else:
             raise ValueError
 
-    def on_dock_closed(self, dock):
-        item = self.dock_to_item[dock]
-        item.applet_dock = None
+    def on_dock_closed(self, item, dock):
         item.applet_geometry = dock.saveGeometry()
         asyncio.ensure_future(dock.terminate())
-        del self.dock_to_item[dock]
         item.setCheckState(0, QtCore.Qt.Unchecked)
 
     def get_untitled(self):