Trying to add dedicated agent to deployment plan, with no agent selected, throws NPE instead of showing user-friendly error

Summary

When dedicating an agent to a deployment plan, if no agent is selected before clicking Add a Null Pointer Exception is thrown instead of a user-friendly error message.

Steps to Reproduce

1. In a deployment plan, go to Agent assignment
2. Under Dedicate agents and images, click Add without selecting an agent

Expected Results

A human readable error like "You must specify an agent or image" should be displayed, similar to the behaviour that occurs when clicking Add on the Required capability section on the same page

Actual Results

A Null Pointer Exception is thrown