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The line popupId = "[default]FFBE/P010/Mixing/Mixer 8_popup" is not doing what you think it's doing. That is just assigning the string value of "[default]FFBE/P010/Mixing/Mixer 8_popup" to the variable popupId, not reading the tag value at that path.
To read a tag value from a script you need to use a system.tag.read* function.