[gnome-maps] mapView: Don't use object when undefined

[Marcus Lundblad <mlundblad src gnome org>]

commit c4c0ede3e8e63fb5af6e7474a19c24dd1e9b919f
Author: Marcus Lundblad <ml update uu se>
Date:   Mon Nov 6 22:13:36 2017 +0100

    mapView: Don't use object when undefined
    
    https://bugzilla.gnome.org/show_bug.cgi?id=782861

 src/mapView.js |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/mapView.js b/src/mapView.js
index 363ae90..fd4f647 100644
--- a/src/mapView.js
+++ b/src/mapView.js
@@ -296,7 +296,7 @@ var MapView = new Lang.Class({
     },
 
     setMapType: function(mapType) {
-        if (this._mapType === mapType)
+        if (this._mapType && this._mapType === mapType)
             return;
 
         let overlay_sources = this.view.get_overlay_sources();