Catastrophic NPE during redraw with show non-conserved and group-based consensus

Problem due to group consensus lines not extending the full width of the alignment.

To reproduce:
1. import feredoxin alignment.
2. create a group that does not cover the full extent of the alignment (say columns 2-45)
3. Select Annotations->Autocalculated->Group Consensus'
4. Select Format->Show nonconserved

NPE like appear on each redraw:
Exception in thread "AWT-EventQueue-2" java.lang.NullPointerException
at jalview.gui.SequenceRenderer.getDisplayChar(SequenceRenderer.java:450)
at jalview.gui.SequenceRenderer.drawText(SequenceRenderer.java:420)
at jalview.gui.SequenceRenderer.drawSequence(SequenceRenderer.java:194)
at jalview.gui.SeqCanvas.draw(SeqCanvas.java:720)
at jalview.gui.SeqCanvas.drawPanel(SeqCanvas.java:646)
at jalview.gui.SeqCanvas.paintComponent(SeqCanvas.java:401)
at javax.swing.JComponent.paint(JComponent.java:1056)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)