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

Fix #9057

app/javascript/mastodon/actions/compose.js

@@ -142,7 +142,7 @@ export function submitCompose(routerHistory) {
         }
       };
 
-      if (response.data.visibility === 'direct' && getState().getIn(['conversations', 'mounted']) <= 0) {
+      if (response.data.visibility === 'direct' && getState().getIn(['conversations', 'mounted']) <= 0 && routerHistory) {
         routerHistory.push('/timelines/direct');
       } else if (response.data.visibility !== 'direct') {
         insertIfOnline('home');

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 = () => {