It shouldn't be possible but it actually happened. I found 3 places where it throws null reference exception - it's in version (2012.1.0614.40):
line 375: this.tipsStack.Clear(); // should be: if (this.tipsStack != null) this.tipsStack.Clear();
line 499: this.popup.IsOpen = false; // should be: if (this.popup != null) this.popup.IsOpen = false;
line 657: this.openedRootVisuals.Add(element); // should be: if (openedRootVisuals != null) this.openedRootVisuals.Add(element);
Sorry, but I really hate this framework. It is the most buggiest code I used. Not because of this bug, but because of the number of bugs you introduce in each new version or hotfix. There is no single version of this framework without problems.
Mr. Mrdec