Normally the name of your application should be read in your .yo-rc.json file (look at the root of your project), it's the "baseName" attribute in that file.
For example, if your application was named "test" when you first ran "yo jhipster", the entity sub-generator should take that name and create a "testApp" AngularJS application.
In your case, it looks like the application name was not read in the .yo-rc.json file: can you check what's inside that file?