Safeguard against null style arg in UserInterface#chooseFile

[imagejan]

This fixes the issue reported by @kephale on the forum, where the absence of a style attribute on a File parameter leads to a NullPointerException.

This PR is just a quick fix, the method should probably be rewritten when addressing #333.

[ctrueden]

Thanks @imagejan. I am concerned that it is possible for style to ever be null. But certainly, being defensive here is a good thing.

[imagejan]

@ctrueden wrote:

I am concerned that it is possible for style to ever be null.

From DefaultWidgetModel, it seems that style being null is allowed/considered:

scijava-common/src/main/java/org/scijava/widget/DefaultWidgetModel.java

Lines 129 to 137 in 821c829

	@Override
	public boolean isStyle(final String style) {
	final String widgetStyle = getItem().getWidgetStyle();
	if (widgetStyle == null) return style == null;
	for (final String s : widgetStyle.split(",")) {
	if (s.equals(style)) return true;
	}
	return false;
	}