Fix bug in multithreaded initialization guard

Author: jholveck

src/mss/linux/xcbhelpers.py

@@ -530,7 +530,7 @@ def initialize(self, callbacks: Iterable[Callable[[], None]] = frozenset()) -> N
             finally:
                 self._initializing = False
 
-            self._initialized = True
+            self.initialized = True
 
 
 LIB = LibContainer()