Remove deleted layers from internal list

There is no use in keeping layers that have been removed from the
project in the internal list.

Fixes #488
Fixes #502
Fixes #468

qfieldsync/gui/layers_config_widget.py

@@ -270,8 +270,11 @@ def _on_message_bus_messaged(self, msg: str) -> None:
         if msg != "layer_config_saved":
             return
 
-        for layer_source in self.layer_sources:
-            layer_source.read_layer()
+        for layer_source in self.layer_sources.copy():
+            try:
+                layer_source.read_layer()
+            except RuntimeError:
+                self.layer_sources.remove(layer_source)
 
         # quite ugly workaround, but this method sometimes operates on deleted objects,
         # so we need to make sure we don't get exceptions