Overview window throws NPE if show boxes is false

If Format | Boxes is unticked, then opening the Overview window causes an NPE to be thrown because the SequenceRenderer returns a null colour:
In 2.10.1:
Exception in thread "Thread-18" java.lang.NullPointerException
at jalview.gui.OverviewPanel.run(OverviewPanel.java:369)
at java.lang.Thread.run(Thread.java:745)

In develop:
Exception in thread "Thread-18" java.lang.NullPointerException
at jalview.renderer.OverviewRenderer.getColumnColourFromSequence(OverviewRenderer.java:158)
at jalview.renderer.OverviewRenderer.draw(OverviewRenderer.java:117)
at jalview.gui.OverviewCanvas.draw(OverviewCanvas.java:130)
at jalview.gui.OverviewPanel.run(OverviewPanel.java:231)
at java.lang.Thread.run(Thread.java:745)