Fix RmlUi mode detection in MainWindowPanel

The check 'SB.view and SB.view.useRmlUi' failed during initialization
because SB.view isn't assigned until AFTER View() constructor completes.

Timeline:
1. View:InitializeRmlUi() runs (inside View constructor)
2. Creates TabbedWindow which calls MainWindowPanel:AddElement()
3. SB.view is STILL NIL (not assigned yet)
4. Check fails, goes to Chili mode path
5. But editors are RmlUi editors (field compat loaded)
6. RmlUi editors don't have .window → crash

Fix: Detect RmlUi mode by checking if RmlUiStringField exists
(which it will if field compat layer is loaded).

Fixes: attempt to index field 'window' (a nil value) at line 131

Author: claude

scen_edit/view/main_window/main_window_panel.lua

@@ -94,8 +94,12 @@ function MainWindowPanel:AddElement(tbl)
     local name = tbl.name
     local editor = tbl.editor
 
+    -- Detect RmlUi mode - check if RmlUi field compat layer is loaded
+    -- (SB.view might not be set yet during initialization)
+    local useRmlUi = RmlUi ~= nil and RmlUiStringField ~= nil
+
     -- In RmlUi mode, skip Chili button creation but still create editors
-    if SB.view and SB.view.useRmlUi then
+    if useRmlUi then
         SB.delay(function()
             if not SB.editors[name] then
                 SB.editors[name] = editor()