Fix NullReferenceException when unregistering folding manager after the document was detached from the TextArea.

Author: dgrunwald

ICSharpCode.AvalonEdit/Rendering/CollapsedLineSection.cs

@@ -86,10 +86,12 @@ public void Uncollapse()
 			if (start == null)
 				return;
 
-			heightTree.Uncollapse(this);
+			if (!heightTree.IsDisposed) {
+				heightTree.Uncollapse(this);
 #if DEBUG
-			heightTree.CheckProperties();
+				heightTree.CheckProperties();
 #endif
+			}
 
 			start = null;
 			end = null;

ICSharpCode.AvalonEdit/Rendering/HeightTree.cs

@@ -73,6 +73,12 @@ public void Dispose()
 			this.weakLineTracker = null;
 		}
 
+		public bool IsDisposed {
+			get {
+				return root == null;
+			}
+		}
+
 		double defaultLineHeight;
 
 		public double DefaultLineHeight {