There is a bug in SimpleViewOnlyStyledModel::lastParagraph() which breaks adding text segments after a Region-based paragraph.

Test cases:

    @Test
    public void addTextAfterRegion() {
        SimpleViewOnlyStyledModel m = new SimpleViewOnlyStyledModel();
        m.addParagraph(() -> new Region());
        m.addNodeSegment(() -> new Region());
        assertEquals(2, m.size());
    }

    @Test
    public void addTextAfterRegionAfterText() {
        SimpleViewOnlyStyledModel m = new SimpleViewOnlyStyledModel();
        m.addNodeSegment(() -> new Region());
        m.addParagraph(() -> new Region());
        m.addSegment("text");
        assertEquals(3, m.size());
    }