add missing kernel_manager in mainwindow.close_tab

IPython/frontend/qt/console/mainwindow.py

@@ -177,6 +177,7 b' class MainWindow(QtGui.QMainWindow):'
                     return
 
         kernel_client = closing_widget.kernel_client
+        kernel_manager = closing_widget.kernel_manager
 
         if keepkernel is None and not closing_widget._confirm_exit:
             # don't prompt, just terminate the kernel if we own it
@@ -238,6 +239,7 b' class MainWindow(QtGui.QMainWindow):'
                 for slave in slave_tabs:
                     background(slave.kernel_client.stop_channels)
                     self.tab_widget.removeTab(self.tab_widget.indexOf(slave))
+                if kernel_manager:
                 kernel_manager.shutdown_kernel()
                 background(kernel_client.stop_channels)