I worked with bete 3 version of Web App Builder and Query widget and it had an error of validating fields.
When we want to ask a value from a user the VALUE field, should be able to commit empty since we ask the value from user. This error from beta 3 and now we have ArcGIs 10.3 released version. (officially release). Still the error is same. ESRI seems do not test their products properly and this is disappointing. Please see the screen capture as below
Portals version of WAB is not the same version that is used on AGOL or WAB developer version. This issue is a know issue and should be fixed in the next release.