public BigInteger getPreviousIndex() {
		return layoutModel.getIndexBefore(lastIndex);
	}
Returns the LayoutModel index of the previous item that was returned via {@link #next()}.

@return index of the previous Layout item returned, or null if this iterator hasn't been
used yet.
Ghidra/Framework/Docking/src/main/java/docking/widgets/fieldpanel/LayoutModelIterator.java