Minor fix in sidebar plugin

Under certain conditions content["files"] is present, but is None,
which led to exception and thus failure of the whole sidebar.

plugins/Sidebar/SidebarPlugin.py

@@ -196,7 +196,7 @@ class UiWebsocketPlugin(object):
         contents = site.content_manager.listContents()  # Without user files
         for inner_path in contents:
             content = site.content_manager.contents[inner_path]
-            if "files" not in content:
+            if "files" not in content or content["files"] is None:
                 continue
             for file_name, file_details in content["files"].items():
                 size_total += file_details["size"]