Steps to reproduce:
1. Start JMC.
2. Open a flight recording. Now the automated rule analysis result page should be displayed.
3. Find another recording in the file system, and drag the new recording to JMC and drop it on top of the analysis result page.
==> JMC crashes.
This was found on macOS 10.12.4 with JMC 6.0.0-663 in JDK 9 build 168.