classes derived from WindowProxy are responsible for their own window creation in ::set_state().

Fixes a crash reported by A.Prokoudine when opening a session with visible plugin GUIs

libs/gtkmm2ext/window_proxy.cc

@@ -97,6 +97,8 @@ WindowProxy::set_state (const XMLNode& node, int /* version */)
 
 		XMLProperty* prop;
 
+		std::cerr << " PB setting state\n";
+		
 		if ((prop = (*i)->property (X_("visible"))) != 0) {
 			_visible = PBD::string_is_affirmative (prop->value ());
 		}
@@ -115,14 +117,6 @@ WindowProxy::set_state (const XMLNode& node, int /* version */)
 		}
 	}
 
-	/* if the window is marked visible but doesn't yet exist, create it */
-	
-	if (_visible) {
-		if (!_window) {
-			_window = get (true);
-		}
-	}
-	
 	if (_window) {
 		setup ();
 	}