Add exception when animation frame could not be found

src/Drivers/Gd/Core.php

@@ -3,6 +3,7 @@
 namespace Intervention\Image\Drivers\Gd;
 
 use Intervention\Image\Collection;
+use Intervention\Image\Exceptions\AnimationException;
 use Intervention\Image\Interfaces\CoreInterface;
 use Intervention\Image\Interfaces\FrameInterface;
 
@@ -31,7 +32,13 @@ class Core extends Collection implements CoreInterface
 
     public function frame(int $position): FrameInterface
     {
-        return $this->getAtPosition($position);
+        $frame = $this->getAtPosition($position);
+
+        if (!($frame instanceof FrameInterface)) {
+            throw new AnimationException('Frame #' . $position . ' could not be found in the image.');
+        }
+
+        return $frame;
     }
 
     public function loops(): int