Prevent custom trigger without container

docs/modules/Assets.md

@@ -337,7 +337,7 @@ editor.on('asset:custom', props => {
     // props.close (Function) - A callback to close the Asset Manager
     // props.remove (Function<Asset>) - A callback to remove an asset
     // props.select (Function<Asset, boolean>) - A callback to select an asset
-    // props.container (HTMLElement) - The element where you can append your UI
+    // props.container (HTMLElement) - The element where you should append your UI
 
     // Here you would put the logic to render/update your UI.
 });

src/asset_manager/index.js

@@ -119,6 +119,9 @@ export default () => {
 
     __trgCustom: debounce(function() {
       const bhv = this.__getBehaviour();
+      if (!bhv.container && !this.getConfig('custom').open) {
+        return;
+      }
       this.em.trigger(this.events.custom, {
         am: this,
         open: this.isOpen(),