block builder: add test to check if the widget exists

src/core/BuilderBlock/BuilderBlockContainer.php

@@ -20,6 +20,11 @@ class BuilderBlockContainer
         return $this->widgets;
     }
 
+    public function hasWidget(string $name)
+    {
+        return isset($this->widgets[$name]);
+    }
+
     public function getWidget(string $name): BuilderBlock
     {
         return $this->widgets[$name];

src/core/Twig/Extension/BuilderExtension.php

@@ -30,6 +30,10 @@ class BuilderExtension extends AbstractExtension
         }
 
         if (isset($data['widget'])) {
+            if (!$this->container->hasWidget($data['widget'])) {
+                return '';
+            }
+
             $widget = $this->container->getWidget($data['widget']);
             $widget->buildVars($data);