Fix non-file page edit error

Signed-off-by: Justin Georgi <justin.georgi@gmail.com>

includes/GlModelViewer.php

@@ -43,7 +43,7 @@ class GlModelViewer extends ImageHandler {
 
   public static function onAlternateEditPreview( EditPage $editor, Content $content, string &$previewHTML, ?ParserOutput &$parserOutput ) {
     $file = MediaWikiServices::getInstance()->getRepoGroup()->findFile($editor->getTitle());
-    if ($file->getMimeType() !== 'model/gltf-binary') {
+    if (!$file || $file->getMimeType() !== 'model/gltf-binary') {
       return true;
     }
     $out = $editor->getContext()->getOutput();