Fixed crash when adding new/existing projects to an existing solution.

src/Main/Base/Project/Src/Project/AbstractProject.cs

@@ -186,6 +186,8 @@ namespace ICSharpCode.SharpDevelop.Project
 			get { return activeConfiguration; }
 			set {
 				WorkbenchSingleton.AssertMainThread();
+				if (value == null)
+					throw new ArgumentNullException();
 				
 				if (activeConfiguration != value) {
 					activeConfiguration = value;
@@ -210,6 +212,8 @@ namespace ICSharpCode.SharpDevelop.Project
 			get { return activePlatform; }
 			set {
 				WorkbenchSingleton.AssertMainThread();
+				if (value == null)
+					throw new ArgumentNullException();
 				
 				if (activePlatform != value) {
 					activePlatform = value;

src/Main/Base/Project/Src/Project/MSBuildBasedProject.cs

@@ -402,9 +402,6 @@ namespace ICSharpCode.SharpDevelop.Project
 				Dictionary<string, string> globalProps = new Dictionary<string, string>();
 				InitializeMSBuildProjectProperties(globalProps);
 				globalProps["Configuration"] = configuration;
-				
-				//HACK: the ActivePlatform property should be set properly before entering here, but sometimes it does not
-				if (platform != null)
 				globalProps["Platform"] = platform;
 				MSBuild.Project project = MSBuildInternals.LoadProject(projectCollection, projectFile, globalProps);
 				if (openCurrentConfiguration)
@@ -1246,7 +1243,9 @@ namespace ICSharpCode.SharpDevelop.Project
 		{
 			this.projectCollection = loadInformation.ParentSolution.MSBuildProjectCollection;
 			this.FileName = loadInformation.FileName;
+			if (loadInformation.Configuration != null)
 				this.ActiveConfiguration = loadInformation.Configuration;
+			if (loadInformation.Platform != null)
 				this.ActivePlatform = loadInformation.Platform;
 			
 			projectFile = ProjectRootElement.Open(loadInformation.FileName, projectCollection);