Fix scene loader

src/Loading/sceneLoader.ts

@@ -368,7 +368,7 @@ export class SceneLoader {
             plugin = <any>registeredPlugin.plugin;
         }
 
-        if (plugin) {
+        if (!plugin) {
             throw "The loader plugin corresponding to the file type you are trying to load has not been found. If using es6, please import the plugin you wish to use before.";
         }