When modal property does not yet exist, create it.

Products/CMFPlone/controlpanel/browser/actions.py

@@ -1,6 +1,7 @@
 from plone.autoform.form import AutoExtensibleForm
 from plone.base.interfaces import IActionSchema
 from plone.base.interfaces import INewActionSchema
+from plone.base.utils import base_hasattr
 from Products.CMFCore.ActionInformation import Action
 from Products.CMFCore.interfaces import IAction
 from Products.CMFCore.interfaces import IActionCategory
@@ -169,7 +170,15 @@ def get_modal(self):
         return self.context.modal
 
     def set_modal(self, value):
-        self.context.modal = value
+        # This property may not exist yet on the context.
+        if not self.context.hasProperty("modal"):
+            if base_hasattr(self.context, "modal"):
+                # We cannot define a property when an attribute with the same
+                # name already exists.
+                delattr(self.context, "modal")
+            self.context._setProperty('modal', value, 'string')
+        else:
+            self.context._setPropValue('modal', value)
 
     modal = property(get_modal, set_modal)