Fix ctrl nil access in _SetFieldVisible for RmlUi mode

RmlUi fields don't have .ctrl property (they're not Chili controls).
Added nil check in _SetFieldVisible() to skip visibility operations
when ctrl doesn't exist.

Fixes: attempt to index local 'ctrl' (a nil value) at editor.lua:290

Author: claude

scen_edit/view/editor.lua

@@ -285,7 +285,12 @@ function Editor:_SetFieldVisible(name, visible)
         return
     end
 
+    -- Skip in RmlUi mode - fields don't have Chili controls
     local ctrl = field.ctrl
+    if not ctrl then
+        return
+    end
+
     --if ctrl.visible ~= visible then
     if ctrl._visible ~= visible then
         if visible then