New instability with #322

[tonydroy]

Coming with changes #322 there is a new problem with saving and reopening documents. Using the most recent (5/27/24) version of RichTextArea, consider the setup from my #315 -- that is, take the FullFeatureDemo equipped with buttons to get and then to open a document. Keep the "action command" version of set.

Start the demo, open the document (in my example, "this is\na test"). Delete the text, press 'update' and then 'set'. The result is a null pointer exception from PieceTable.java:359 because "prevDecoration" is null.

I am getting this crash all over in an attempted project which routinely saves and opens empty documents.

(PS. I do not consider myself a "computer person" and if I am doing something wrong, or there is something inadequate or inappropriate about my posts, I would appreciate finding out!)