plugins: don't access property on uninitialized plugin

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

poezio/plugin_manager.py

@@ -158,6 +158,7 @@ class PluginManager:
 
         if name in self.plugins:
             try:
+                if self.plugins[name] is not None:
                     self.plugins[name]._unloading = True  # Prevents loops
                     for rdep in self.rdeps[name].copy():
                         if rdep in self.plugins and not self.plugins[rdep]._unloading: