DefaultWriter text draw outside screen causing NPE

[jvalkeal]

For example npe may come from here:

spring-shell/spring-shell-core/src/main/java/org/springframework/shell/component/view/screen/DefaultScreen.java

Lines 302 to 303 in 9830faf

	DefaultScreenItem item = layer.getScreenItem(x + i, y);
	item.content = Character.toString(c);

Need to sweep through other similar places where screen item may be null.