Fix +10613 don't crash when there is not a next panel

plugins/c9.ide.editors/pane.js

@@ -585,6 +585,7 @@ define(function(require, module, exports) {
                     next = next.selectNodes("tab|hsplitbox|vsplitbox")[op]();
                 }
                 
+                if (next) {
                     // move all pages to another pane
                     getTabs().forEach(function(tab) {
                         tab.attachTo(next.cloud9pane, null, true);
@@ -612,6 +613,7 @@ define(function(require, module, exports) {
                     else {
                         queue = [parent];
                     }
+                }
                 
                 if (last && last.parentNode.localName == "bar") {
                     last.$ext.style.width = "100%";//setAttribute("width", "100%");