Fix facade::$app could be nullable

src/Facade/Pdf.php

@@ -4,6 +4,7 @@
 
 use Barryvdh\DomPDF\PDF as BasePDF;
 use Illuminate\Support\Facades\Facade as IlluminateFacade;
+use RuntimeException;
 
 /**
  * @method static BasePDF setBaseHost(string $baseHost)
@@ -47,9 +48,15 @@ protected static function getFacadeAccessor()
      * @param string $method
      * @param array<mixed> $args
      * @return mixed
+     *
+     * @throws \RuntimeException
      */
     public static function __callStatic($method, $args)
     {
+        if (! static::$app) {
+            throw new RuntimeException('Facade application has not been set.');
+        }
+
         $instance = static::$app->make(static::getFacadeAccessor());
 
         return $instance->$method(...$args);