Official way for handling exception in GWT Designer is creating error report (as suggested by error page) and creating new issue, with report attached.
In other case this will lead to discussions "give us this file", "show log", etc.
Be wise and attach enough information, which may be useful, such as module.gwt.xml files.
2. Yes, GWT Designer supports @UiField(provided=true). But as you understand, there are no magic. GWT Designer can not know that this interface should be replaced with that class.
So, you should help it, by writing script to create instance.
See for example CellList.wbp-component.xml in GWT Designer plugin (in wbp-meta folder).
<parameter name="UiBinder.createInstance"><![CDATA[
import com.google.gwt.cell.client.TextCell;
import com.google.gwt.user.cellview.client.*;
list = new CellList(new TextCell());
list.setRowData(0, {'1. Item', '2. Long item', '3. Even longer item', '4. Item', '5. Long item'});
list.setRowCount(5);
return list;
]]></parameter>