Fix SD-1767 - NullReferenceException when closing a solution while "Loading References..." is active.

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

@@ -407,6 +407,9 @@ namespace ICSharpCode.SharpDevelop.Project
 			try {
 				System.Threading.Monitor.Enter(this.SyncRoot, ref lockTaken);
 				
+				if (projectFile == null)
+					throw new ObjectDisposedException("MSBuildBasedProject");
+				
 				if (configuration == null)
 					configuration = this.ActiveConfiguration;
 				if (platform == null)

src/Main/Base/Project/Src/Services/ParserService/ParseProjectContent.cs

@@ -66,7 +66,7 @@ namespace ICSharpCode.SharpDevelop
 					if (reference != null) {
 						// TODO: Translate me
 //						progressMonitor.TaskName = "Loading " + reference.ShortName + "...";
-						AddReference(reference, false);
+						AddReference(reference, false, progressMonitor.CancellationToken);
 					}
 				}
 			}
@@ -109,7 +109,7 @@ namespace ICSharpCode.SharpDevelop
 			}
 		}
 		
-		void AddReference(ReferenceProjectItem reference, bool updateInterDependencies)
+		void AddReference(ReferenceProjectItem reference, bool updateInterDependencies, CancellationToken cancellationToken)
 		{
 			try {
 				AddReferencedContent(AssemblyParserService.GetProjectContentForReference(reference));
@@ -122,6 +122,13 @@ namespace ICSharpCode.SharpDevelop
 				// If the user removes the reference and then re-adds it, there might be other references
 				// in the project depending on it, so we do the refresh after the old reference was added.
 				AssemblyParserService.RefreshProjectContentForReference(reference);
+			} catch (OperationCanceledException) {
+				throw;
+			} catch (ObjectDisposedException e) {
+				// ObjectDisposedException can happen if project gets disposed while LoadSolutionProjectsThread is running.
+				// We will ignore the ObjectDisposedException and throw OperationCanceledException instead.
+				cancellationToken.ThrowIfCancellationRequested();
+				MessageService.ShowException(e);
 			} catch (Exception e) {
 				MessageService.ShowException(e);
 			}