Fix JS error when posting from page without router context (#9073)

Fix #9057

app/javascript/mastodon/features/compose/components/compose_form.js

@@ -88,7 +88,7 @@ class ComposeForm extends ImmutablePureComponent {
       return;
     }
 
-    this.props.onSubmit(this.context.router.history);
+    this.props.onSubmit(this.context.router ? this.context.router.history : null);
   }
 
   onSuggestionsClearRequested = () => {