Fix process-node-related bugs (#967)

This PR handles a couple of bugs related to the process node, specifically loading node views and managing the process state.

src/aiidalab_qe/app/result/__init__.py

@@ -189,6 +189,8 @@ def _update_node_view(self, nodes, refresh=False):
             self.node_view_container.children = [self.node_view_loading_message]
             self.node_view = self._create_workchain_viewer(node)
             self.node_views[node.uuid] = self.node_view
+        else:
+            self.node_view = ipw.HTML("No viewer available for this node.")
 
         self.node_view_container.children = [self.node_view]
 

src/aiidalab_qe/common/panel.py

@@ -556,7 +556,11 @@ def _get_child_process_status(self, child="this"):
         """
 
     def _get_child_state_and_exit_message(self, child="this"):
-        if not (node := self._fetch_child_process_node(child)):
+        if not (
+            (node := self._fetch_child_process_node(child))
+            and hasattr(node, "process_state")
+            and node.process_state
+        ):
             return "queued", None
         if node.is_failed:
             return "failed", node.exit_message