fix media removal

backend/app/Controllers/ElementController.php

@@ -114,7 +114,15 @@ class ElementController
 
     private function stringInput(Request $request, string $key): ?string
     {
+        if (! $request->exists($key)) {
+            return null;
+        }
+
         $value = $request->input($key);
+        if ($value === null) {
+            return '';
+        }
+
         if (! is_string($value)) {
             return null;
         }