I just got this bug when I went to build a wooden mine support. No clue what caused it but I thought that I should put it here.
java.lang.NullPointerException
at Link.mkpreview(consobj-pv.cjava:112)
at Link.updowner(consobj-pv.cjava:128)
at Link.tick(consobj-pv.cjava:193)
at haven.Widget.tick(Widget.java:637)
at haven.Widget.tick(Widget.java:637)
at haven.GameUI.tick(GameUI.java:683)
at haven.Widget.tick(Widget.java:637)
at haven.UI.tick(UI.java:135)
at haven.HavenPanel.run(HavenPanel.java:540)
at java.lang.Thread.run(Unknown Source)
Edit: I was using the official client.