In the file https://github.com/Teiid-Designer/teiid-designer/blob/9.0.x/plugins/org.teiid.designer.datatools/src/org/teiid/designer/datatools/profiles/ws/WSConnectionInfoProvider.java

@line 101:
if (null != user &! user.equals(""))

It is not correctly captured that user can be null. No short-circuit operator is used and therefore, the right-hand side of the expression is evaluated which gives a NullPointer if user is null.

Line 101 contains ampersand exclamation mark without a space in between such that short-circuit is not working. ( if (null != user &! user.equals("")) { )
This should be ( if (null != user && !user.equals("")) { )

After importing a REST Web Service to create a Source and View model, previewing the data generates the following NullPointer.

java.lang.NullPointerException
at org.teiid.designer.datatools.profiles.ws.WSConnectionInfoProvider.getTeiidRelatedProperties(WSConnectionInfoProvider.java:101)
at org.teiid.designer.runtime.TeiidDataSourceFactory.createDataSource(TeiidDataSourceFactory.java:127)
at org.teiid.designer.runtime.preview.PreviewManager.ensureConnectionInfoIsValid(PreviewManager.java:527)
at org.teiid.designer.runtime.preview.PreviewManager.previewSetup(PreviewManager.java:1365)
at org.teiid.designer.runtime.ui.preview.PreviewDataWorker$1.run(PreviewDataWorker.java:233)
at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:122)