When collection has no vars then dont try to collect them

(cherry picked from commit ee7528186f69e811b1b4db8e69ea2e4ec0ea2613)

src/Exporters/Abstract/AbstractConverter.php

@@ -71,7 +71,7 @@ abstract class AbstractConverter implements ConverterContract
      */
     protected function setVariables(): static
     {
-        if ($this->collection->variable) {
+        if (isset($this->collection?->variable)) {
             foreach ($this->collection->variable as $var) {
                 $this->vars["{{{$var->key}}}"] = $var->value;
             }