Fix some more crashes reported by UDC.

src/AddIns/DisplayBindings/AvalonEdit.AddIn/Src/CodeEditor.cs

@@ -386,6 +386,8 @@ namespace ICSharpCode.AvalonEdit.AddIn
 		
 		void TextAreaCaretPositionChanged(object sender, EventArgs e)
 		{
+			if (document == null)
+				return; // can happen if the editor is closed with Ctrl+F4 while selecting text
 			Debug.Assert(sender is Caret);
 			Debug.Assert(!document.IsInUpdate);
 			if (sender == this.ActiveTextEditor.TextArea.Caret) {

src/Main/Base/Project/Src/Gui/Pads/PropertyPad/PropertyPad.cs

@@ -356,9 +356,9 @@ namespace ICSharpCode.SharpDevelop.Gui
 		
 		void ComboBoxSelectedIndexChanged(object sender, EventArgs e)
 		{
-			if (!inUpdate) {
-				if (host!=null) {
+			if (!inUpdate && host!=null) {
 				ISelectionService selectionService = (ISelectionService)host.GetService(typeof(ISelectionService));
+				if (selectionService != null) {
 					if (comboBox.SelectedIndex >= 0) {
 						selectionService.SetSelectedComponents(new object[] { GetComboBoxItem(comboBox.SelectedIndex) });
 					} else {

src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/ReflectionLayer/DomPersistence.cs

@@ -38,6 +38,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 		#region Cache management
 		public string SaveProjectContent(ReflectionProjectContent pc)
 		{
+			try {
 				// create cache directory, if necessary
 				Directory.CreateDirectory(cacheDirectory);
 				
@@ -52,6 +53,11 @@ namespace ICSharpCode.SharpDevelop.Dom
 					WriteProjectContent(pc, fs);
 				}
 				return fileName;
+			} catch (IOException) {
+				return null;
+			} catch (UnauthorizedAccessException) {
+				return null;
+			}
 		}
 		
 		public ReflectionProjectContent LoadProjectContentByAssemblyName(string assemblyName)