Exception in NavigationItemListCellRenderer$LeftRenderer.customizeCellRenderer

I get this exception when using CtrlN with my custom language's "go to class" provider. I don't know what it means. It just started happening; CtrlN has worked for a while.

What am I doing wrong?

java.lang.NullPointerException
at com.intellij.ide.util.NavigationItemListCellRenderer$LeftRenderer.customizeCellRenderer(NavigationItemListCellRenderer.java:17)
at com.intellij.ui.ColoredListCellRenderer.getListCellRendererComponent(ColoredListCellRenderer.java:49)
at com.intellij.ide.util.NavigationItemListCellRenderer.getListCellRendererComponent(NavigationItemListCellRenderer.java:7)
at javax.swing.plaf.basic.BasicListUI.updateLayoutState(BasicListUI.java:1148)
at javax.swing.plaf.basic.BasicListUI.maybeUpdateLayoutState(BasicListUI.java:1098)
at javax.swing.plaf.basic.BasicListUI.locationToIndex(BasicListUI.java:714)
at javax.swing.JList.locationToIndex(JList.java:1144)
at javax.swing.JList.getLastVisibleIndex(JList.java:898)
at com.intellij.ui.ListScrollingUtil.getVisibleRowCount(ListScrollingUtil.java:151)
at com.intellij.ui.ListScrollingUtil.ensureIndexIsVisible(ListScrollingUtil.java:123)
at com.intellij.ui.ListScrollingUtil.selectItem(ListScrollingUtil.java:29)
at com.intellij.ide.util.gotoByName.ChooseByNameBase$ListUpdater$1.run(ChooseByNameBase.java:4)
at com.intellij.util.Alarm$1.run(Alarm.java:96)
at com.intellij.util.Alarm$MyThread$1.run(Alarm.java:238)
at com.intellij.openapi.application.impl.LaterInvocatorEx$FlushQueue.run(LaterInvocatorEx.java:16)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:461)
at com.intellij.ide.IdeEventQueue.defaultDispatchEvent(IdeEventQueue.java:65)
at com.intellij.ide.IdeEventQueue.a(IdeEventQueue.java:56)
at com.intellij.ide.IdeEventQueue.dispatchEvent(IdeEventQueue.java:160)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)