Fenix activation explore breaks if measures from two different labeled_counters are selected

[scholtzan]

See https://mozilla.cloud.looker.com/explore/fenix/activation?qid=3KnbWKRx60cUq13Pg570FX

invalidQuery: No matching signature for operator - for argument types: STRING, STRING. Supported signatures: INT64 - INT64; NUMERIC - NUMERIC; BIGNUMERIC - BIGNUMERIC; FLOAT64 - FLOAT64; DATE - INT64; DATE - DATE; TIMESTAMP - TIMESTAMP; DATETIME - DATETIME; TIME - TIME; TIMESTAMP - INTERVAL; DATE - INTERVAL; DATETIME - INTERVAL; INTERVAL - INTERVAL at [5:217]

If measures from different Glean Errors are selected in a single query, then Looker is showing an error. Looking at the SQL, the raw query gets changed to something strange. I wonder if it's related to the dataset having mostly no values in the labeled counters.