Fix NullReferenceException when the debugger loads a module while no solution is loaded.

src/AddIns/Debugger/Debugger.AddIn/Service/WindowsDebugger.cs

@@ -720,6 +720,9 @@ namespace ICSharpCode.SharpDevelop.Services
 		
 		void process_Modules_Added(object sender, CollectionItemEventArgs<Module> e)
         {
+			if (ProjectService.OpenSolution == null)
+				return;
+			
             ProjectService.OpenSolution.Projects
                 .Where(p => e.Item.Name.IndexOf(p.Name) >= 0)
             	.ForEach(p => e.Item.LoadSymbolsFromDisk(new []{ Path.GetDirectoryName(p.OutputAssemblyFullPath) }));