[plugin_installer] Handle case when no plugins are available

plugin_installer/plugin_installer.py

@@ -161,6 +161,9 @@ class PluginInstaller(GajimPlugin):
             return
 
         plugin_list = parse_manifests_zip(data)
+        if not plugin_list:
+            log.warning('No plugins found in zip')
+
         if self._available_page is None:
             return
         self._available_page.append_plugins(plugin_list)

plugin_installer/widget.py

@@ -58,7 +58,10 @@ class AvailablePage(Observable):
     def append_plugins(self, plugins):
         for plugin in plugins:
             self._ui.plugin_store.append(plugin.fields)
-        self._select_first_plugin()
+
+        if plugins:
+            self._select_first_plugin()
+
         self._update_install_button()
         self._ui.spinner.stop()
         self._ui.spinner.hide()