Use isinstance to verify type

[JCZuurmond]

make fmt is failing on this

[github-actions[bot]]

✅ 35/35 passed, 3 flaky, 2 skipped, 10m8s total

Flaky tests:

• 🤪 test_dashboard_deploys_dashboard_the_same_as_created_dashboard (9.135s)
• 🤪 test_dashboard_deploys_dashboard_with_counter_variation (16.497s)
• 🤪 test_dashboards_deploys_exported_dashboard_definition (17.192s)

_{Running from acceptance #321}

[JCZuurmond]

Other failures are resolved in #226