I've submitted this bug via the "contact support" button, but I'm not sure if there is a way to see the status of the support submittal.
So it appears if you have the 'Prefix field access with "this"' option turned on on the Code Generation preferences page the designer's parser might give you this:
Exception during constructor evaluation
An exception happened during evaluation of
new Label(this.shell, SWT.NONE)
Constructor Label(Composite,int) was used with the parameters {<null>,0}.
Note: null values may occur when parsing complex expressions that can't be resolved at design-time.
com.instantiations.designer.core.utils.exception.DesignerException: 2005 (Null as parent argument.).
I did a clean install of the latest (not beta) plugins in a helios setup and use the following to produce this:
1) Create a new SWT Application window with default options
2) Via designer I try dropping a label on to the shell.
The designer has no issues if I turn off the prefix option and I manually remove all of the "this." prefixes (for existing classes that i edit with the designer). This issue also occurred when I used the beta plugins as well.