Fix exception when there is a file conflict installing jQuery.

Always use UI thread when opening FileConflictResolver dialog
when running PowerShell scripts.

src/AddIns/Misc/PackageManagement/Project/Src/FileConflictResolver.cs

@@ -9,13 +9,29 @@ namespace ICSharpCode.PackageManagement
 {
 	public class FileConflictResolver : ServiceWithWorkbenchOwner, IFileConflictResolver
 	{
+		IPackageManagementWorkbench workbench;
+		
+		public FileConflictResolver()
+			: this(new PackageManagementWorkbench())
+		{
+		}
+		
+		public FileConflictResolver(IPackageManagementWorkbench workbench)
+		{
+			this.workbench = workbench;
+		}
+		
 		public FileConflictResolution ResolveFileConflict(string message)
 		{
+			if (workbench.InvokeRequired) {
+				return workbench.SafeThreadFunction(() => ResolveFileConflict(message));
+			} else {
 				var viewModel = new FileConflictViewModel(message);
 				FileConflictView view = CreateFileConflictView(viewModel);
 				view.ShowDialog();
 				return viewModel.GetResolution();
 			}
+		}
 		
 		FileConflictView CreateFileConflictView(FileConflictViewModel viewModel)
 		{