check if both target and subscriptions are defined before accessing

[freilich-am]

Description

Please explain the changes you made here. Commit your changes with npm run commit instead of git commit.

Specify if it's a fix, feature or breaking change to update the version on NPM.

Thank you!

Checklist

Please, check that you have been followed next steps:

• I've read the (contributing)[https://github.com/videogular/videogular2/blob/master/CONTRIBUTING.md] guidelines
• Code compiles correctly (run npm start)
• Created tests (if necessary)
• All tests passing (run npm test)
• Extended the README / documentation (if necessary)