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.