SetDefaultsWidget: Don't crash on unknown custom keys

Just warn about it in the logs.
This will only show up when opening the advanced settings UI, so we'll
keep it for now instead of *also* deleting the offending kv pair,
for the sake of not randomly destroying user content...

frontend/apps/filemanager/filemanagersetdefaults.lua

@@ -57,8 +57,15 @@ function SetDefaultsWidget:init()
     end
 
     for k, v in pairs(rw_defaults) do
-        self.state[k].value = v
-        self.state[k].custom = true
+        -- Warn if we encounter a deprecated (or unknown) customized key
+        if not self.state[k] then
+            logger.warn("G_defaults: Found an unknown key in custom settings:", k)
+            -- Should we just delete it?
+            --G_defaults:delSetting(k)
+        else
+            self.state[k].value = v
+            self.state[k].custom = true
+        end
     end
 
     -- Prepare our menu entires