Avoid NullReferenceException when DataObject.GetData returns null.

src/Main/Base/Project/Src/Gui/Workbench/WpfWorkbench.cs

@@ -627,12 +627,14 @@ namespace ICSharpCode.SharpDevelop.Gui
 			try {
 				if (data != null && data.GetDataPresent(DataFormats.FileDrop)) {
 					string[] files = (string[])data.GetData(DataFormats.FileDrop);
+					if (files != null) {
 						foreach (string file in files) {
 							if (File.Exists(file)) {
 								return DragDropEffects.Link;
 							}
 						}
 					}
+				}
 			} catch (COMException) {
 				// Ignore errors getting the data (e.g. happens when dragging attachments out of Thunderbird)
 			}
@@ -646,7 +648,8 @@ namespace ICSharpCode.SharpDevelop.Gui
 				if (!e.Handled && e.Data != null && e.Data.GetDataPresent(DataFormats.FileDrop)) {
 					e.Handled = true;
 					string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
-					
+					if (files == null)
+						return;
 					foreach (string file in files) {
 						if (File.Exists(file)) {
 							Project.IProjectLoader loader = Project.ProjectService.GetProjectLoader(file);