fixed Passing null to parameter

Closes #3421080

src/ColorSystemBrandingBlockAlter.php

@@ -24,7 +24,7 @@ class ColorSystemBrandingBlockAlter implements RenderCallbackInterface {
 
     // Override logo.
     $logo = $config->get('logo');
-    if ($logo && $build['content']['site_logo'] && preg_match('!' . $theme_key . '/logo.svg$!', $build['content']['site_logo']['#uri'])) {
+    if ($logo && isset($build['content']['site_logo']['#uri']) && preg_match('!' . $theme_key . '/logo.svg$!', $build['content']['site_logo']['#uri'])) {
       $build['content']['site_logo']['#uri'] = \Drupal::service('file_url_generator')->generateString($logo);
     }