Disallow join button without IP address Input

Closes #20234

Trying to join a server with an empty address crashes the game. This fix disallows pressing join button without ip address field input.  Updated to reuse search for joinButton vs 2 separate calls to search.

OpenRA.Mods.Common/Widgets/Logic/DirectConnectLogic.cs

@@ -39,9 +39,11 @@ public DirectConnectLogic(Widget widget, Action onExit, Action openLobby, Connec
 				portField.Text = text.Substring(last + 1);
 			}
 
-			panel.Get<ButtonWidget>("JOIN_BUTTON").IsDisabled = () => string.IsNullOrEmpty(ipField.Text);
+			var joinButton = panel.Get<ButtonWidget>("JOIN_BUTTON");
 
-			panel.Get<ButtonWidget>("JOIN_BUTTON").OnClick = () =>
+			joinButton.IsDisabled = () => string.IsNullOrEmpty(ipField.Text);
+
+			joinButton.OnClick = () =>
 			{
 				var port = Exts.WithDefault(1234, () => Exts.ParseIntegerInvariant(portField.Text));