While I was developing a Web Client module I got this error. This happens because of a dependency issue.

I had used the dependency

<dependency>
            <groupId>org.apache.cxf</groupId>
            <artifactId>cxf-rt-frontend-simple</artifactId>
            <version>3.0.2</version>
</dependency>

This got me the error continuously. The one you need is

 <dependency>
            <groupId>org.apache.cxf</groupId>
            <artifactId>cxf-rt-frontend-jaxws</artifactId>
            <version>3.0.2</version><</dependency>

Instead of simple, you have to use jaxws. That's it :) this saved me. (Though it is so simple)