Fix crash due to load invalid preference.xml

SourceGit/App.xaml

@@ -2,8 +2,7 @@
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              Startup="OnAppStartup" 
-             Deactivated="OnAppDeactivated"
-             Exit="OnAppExit">
+             Deactivated="OnAppDeactivated">
     <Application.Resources>
         <ResourceDictionary>
             <ResourceDictionary.MergedDictionaries>

SourceGit/App.xaml.cs

@@ -119,16 +119,8 @@ namespace SourceGit {
         /// <param name="sender"></param>
         /// <param name="e"></param>
         private void OnAppDeactivated(object sender, EventArgs e) {
-            GC.Collect();
-        }
-
-        /// <summary>
-        ///     Quit event.
-        /// </summary>
-        /// <param name="sender"></param>
-        /// <param name="e"></param>
-        private void OnAppExit(object sender, ExitEventArgs e) {
             Git.Preference.Save();
+            GC.Collect();
         }
     }
 }

SourceGit/Git/Preference.cs

@@ -134,10 +134,14 @@ namespace SourceGit.Git {
                 return;
             }
 
+            try {
                 var stream = new FileStream(SAVE_PATH, FileMode.Open);
                 var reader = new XmlSerializer(typeof(Preference));
                 instance = (Preference)reader.Deserialize(stream);
                 stream.Close();
+            } catch {
+                instance = new Preference();
+            }            
         }
 
         /// <summary>

SourceGit/Git/Repository.cs

@@ -4,7 +4,6 @@ using System.Diagnostics;
 using System.IO;
 using System.Text;
 using System.Text.RegularExpressions;
-using System.Windows.Forms;
 using System.Windows.Threading;
 using System.Xml.Serialization;