Issue #3487475 by mably: Warning in logs when updating ExcelExport display settings

Closes #3487475

src/Plugin/views/display/ExcelExport.php

@@ -294,7 +294,10 @@ class ExcelExport extends RestExport {
     }
 
     // Uppercase the background color RGB values.
-    $form_state->setValue('header_background_color', strtoupper($form_state->getValue('header_background_color')));
+    $header_background_color = $form_state->getValue('header_background_color');
+    if ($header_background_color !== NULL && $header_background_color !== '') {
+      $form_state->setValue('header_background_color', strtoupper($header_background_color));
+    }
   }
 
   /**