fix: occasional AttributeError in CythonEvent.__repr__ (#487)

a_sync/primitives/locks/event.pyx

@@ -82,9 +82,18 @@ cdef class CythonEvent(_DebugDaemonMixin):
             if self.__name
             else "object"
         )
+
         cdef str status = "set" if self._value else "unset"
-        if self._waiters:
-            status += ", waiters:{}".format(len(self._waiters))
+
+        try:
+            waiters = self._waiters
+        except AttributeError:
+            # TODO: debug how this error is possible since _waiters is set in __init__
+            pass
+        else:
+            if len_waiters := len(waiters):
+                status += ", waiters:{}".format(len_waiters)
+
         return "<{}.{} {} at {} [{}]>".format(
             self.__class__.__module__, 
             self.__class__.__name__, 
@@ -185,4 +194,4 @@ cdef class CythonEvent(_DebugDaemonMixin):
 
 
 cdef inline bint _is_not_done(fut: asyncio.Future):
-    return <str>fut._state == "PENDING"
+    return <str>fut._state == "PENDING"