diff --git a/python/mmSolver/ui/base_maya_window.py b/python/mmSolver/ui/base_maya_window.py
index a44859203..8af8193fd 100644
--- a/python/mmSolver/ui/base_maya_window.py
+++ b/python/mmSolver/ui/base_maya_window.py
@@ -61,6 +61,7 @@ def apply(self):
 
 
 LOG = mmSolver.logger.get_logger()
+ALL_CLASS_INSTANCES = set()
 
 
 class BaseMayaWindow(MayaQWidgetDockableMixin,
@@ -75,6 +76,9 @@ def __init__(self, parent, name=None):
         super(BaseMayaWindow, self).__init__()
         self.windowGeometry = None
 
+        global ALL_CLASS_INSTANCES
+        ALL_CLASS_INSTANCES.add(self)
+
         # Destroy this widget when closed. Otherwise it will stay around.
         self.setAttribute(QtCore.Qt.WA_DeleteOnClose, True)