HPCC-8567 Issues when graph control is missing.

All calls to _plugin should check it exists first.

Signed-off-by: Gordon Smith <gordon.smith@lexisnexis.com>
Merge pull request #3871 from GordonSmith/HPCC-8567-master

HPCC-8567 Issues when graph control is missing.

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>