fix

app/code/Magento/Theme/Block/Html/Topmenu.php

@@ -231,7 +231,15 @@ class Topmenu extends Template implements IdentityInterface
 
             $html .= '</div>';
             $html .= '</div>';
-        } else {
+        } else if($childLevel == 0) {
+            $html .= '<div class="level' . $childLevel . ' ' . $childrenWrapClass . '">';
+            $html .= '<div class="row">';
+            $html .= '<ul class="subchildmenu col-sm-12 mega-columns columns4">';
+            $html .= $this->_getHtmlStyle1($child, $childrenWrapClass, $limit, $colStops);
+            $html .= '</ul>';
+            $html .= '</div>';
+            $html .= '</div>';
+        } else{
             $html .= '<ul class="level' . $childLevel . '">';
             $html .= $this->_getHtml($child, $childrenWrapClass, $limit, $colStops);
             $html .= '</ul>';