RFR: 8262725: IGV: crash when removing all graphs in a group

[Roberto Castañeda Lozano]

This change makes IGV close graph views when their groups are either a) removed or b) emptied (all graphs are removed but the group remains), and avoids faulty graph view computation in case b).

Tested the following scenarios manually (on groups loaded both via network and from a file, and on JDK 8, 11, and 15):
1. open a graph, then remove all graphs in the group (as described in the [bug report](https://bugs.openjdk.java.net/browse/JDK-8262725));
2. open a graph, then remove its group; and
3. open a graph, then remove all graphs and groups (File -> Remove all graphs and groups).

-------------

Commit messages:
 - Close graph views when their groups are removed or emptied

Changes: https://git.openjdk.java.net/jdk/pull/3540/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3540&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262725
  Stats: 33 lines in 2 files changed: 31 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3540.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3540/head:pull/3540

PR: https://git.openjdk.java.net/jdk/pull/3540