Build Plan Filter while creating a TagTrigger is not checked for plan authenticity.

Issue Summary

Build Plan Filter while creating a TagTrigger is not checked for plan authenticity.
When we setup a tag trigger and want to set a plan key filter and set some arbitrary value (not a valid plan key which doesn't seem to a plan key), we get an internal server error.

This is reproducible on Data Center: yes

Steps to Reproduce

1. Create a plan
2. Configure Tag trigger for the plan
3. Under trigger condition provide an invalid plan key and Save trigger

Expected Results

The tag trigger should not be created at all and a valid error message should pop up stating that the "Plan key provided is not valid"

Actual Results

Tag trigger is created and we get an internal server error.

Workaround

Create the tag trigger with a valid plan key in the trigger condition