[gnome-shell] inputMethod: Add a null-check for text in vfunc_set_surrounding.

[Florian Müllner <fmuellner src gnome org>]

commit 0cf2d396b0d584f59402a6a0c42376639bd314ef
Author: Andrea Azzarone <andrea azzarone canonical com>
Date:   Mon Sep 17 18:00:04 2018 +0200

    inputMethod: Add a null-check for text in vfunc_set_surrounding.
    
    Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/579

 js/misc/inputMethod.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/js/misc/inputMethod.js b/js/misc/inputMethod.js
index 320a6cc33..ec84f7277 100644
--- a/js/misc/inputMethod.js
+++ b/js/misc/inputMethod.js
@@ -176,7 +176,7 @@ var InputMethod = new Lang.Class({
     },
 
     vfunc_set_surrounding(text, cursor, anchor) {
-        if (this._context)
+        if (this._context && text)
             this._context.set_surrounding_text(text, cursor, anchor);
     },