[gnome-usage] application: Make sure there's just a single Window instance

[Felipe Borges <felipeborges src gnome org>]

commit 309db9b2f4368e343c01f484fdfcb59c63bb9530
Author: Felipe Borges <felipeborges gnome org>
Date:   Mon Sep 4 16:05:13 2017 +0200

    application: Make sure there's just a single Window instance
    
    https://bugzilla.gnome.org/show_bug.cgi?id=787257

 src/application.vala |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/application.vala b/src/application.vala
index aec27a6..d34aea8 100644
--- a/src/application.vala
+++ b/src/application.vala
@@ -47,6 +47,9 @@ namespace Usage
 
         public override void activate()
         {
+            if (window != null)
+                return;
+
             window = new Window(this);
 
             // Create menu