Added some null ref checks

Throw proper exceptions if trying to use drawing api before
initializing XWT.

Xwt/Xwt.Drawing/Image.cs

@@ -162,6 +162,8 @@ namespace Xwt.Drawing
 				throw new ArgumentNullException ("resource");
 			
 			var toolkit = Toolkit.CurrentEngine;
+			if (toolkit == null)
+				throw new ToolkitNotInitializedException ();
 
 			var name = Path.GetFileNameWithoutExtension (resource);
 
@@ -194,6 +196,8 @@ namespace Xwt.Drawing
 
 		public static Image CreateMultiSizeIcon (IEnumerable<Image> images)
 		{
+			if (Toolkit.CurrentEngine == null)
+				throw new ToolkitNotInitializedException ();
 			return new Image (Toolkit.CurrentEngine.ImageBackendHandler.CreateMultiSizeIcon (images.Select (i => i.GetBackend ())));
 		}
 
@@ -254,12 +258,16 @@ namespace Xwt.Drawing
 		public static Image FromFile (string file)
 		{
 			var toolkit = Toolkit.CurrentEngine;
+			if (toolkit == null)
+				throw new ToolkitNotInitializedException ();
 			return new Image (toolkit.ImageBackendHandler.LoadFromFile (file), toolkit);
 		}
 		
 		public static Image FromStream (Stream stream)
 		{
 			var toolkit = Toolkit.CurrentEngine;
+			if (toolkit == null)
+				throw new ToolkitNotInitializedException ();
 			return new Image (toolkit.ImageBackendHandler.LoadFromStream (stream), toolkit);
 		}
 		

Xwt/Xwt.csproj

@@ -315,6 +315,7 @@
     <Compile Include="Xwt\Keyboard.cs" />
     <Compile Include="Xwt.Backends\KeyboardHandler.cs" />
     <Compile Include="Xwt\FrameBox.cs" />
+    <Compile Include="Xwt\ToolkitNotInitializedException.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <ItemGroup />

Xwt/Xwt/ToolkitNotInitializedException.cs

@@ -0,0 +1,20 @@
+//
+// ToolkitNotInitializedException.cs
+//
+// Author:
+//       Lluis Sanchez <lluis@xamarin.com>
+//
+// Copyright (c) 2013 Xamarin Inc.
+//
+using System;
+
+namespace Xwt
+{
+	public class ToolkitNotInitializedException: Exception
+	{
+		public ToolkitNotInitializedException (): base ("XWT has not been initialized")
+		{
+		}
+	}
+}
+