Thank you for the tip! 
Weird thing. response.data returns:
{"result":{"uv":0,"uv_time":"2021-04-26T18:55:43.988Z","uv_max":3.3945,"uv_max_time":"2021-04-26T11:05:47.055Z","ozone":453.6,"ozone_time":"2021-04-26T18:04:04.915Z","safe_exposure_time":{"st1":null,"st2":null,"st3":null,"st4":null,"st5":null,"st6":null},"sun_info":{"sun_times":{"solarNoon":"2021-04-26T11:05:47.055Z","nadir":"2021-04-25T23:05:47.055Z","sunrise":"2021-04-26T03:36:02.718Z","sunset":"2021-04-26T18:35:31.392Z","sunriseEnd":"2021-04-26T03:40:15.223Z","sunsetStart":"2021-04-26T18:31:18.888Z","dawn":"2021-04-26T02:53:17.460Z","dusk":"2021-04-26T19:18:16.650Z","nauticalDawn":"2021-04-26T01:56:17.438Z","nauticalDusk":"2021-04-26T20:15:16.673Z","nightEnd":"2021-04-26T00:37:31.331Z","night":"2021-04-26T21:34:02.779Z","goldenHourEnd":"2021-04-26T04:28:07.686Z","goldenHour":"2021-04-26T17:43:26.425Z"},"sun_position":{"azimuth":2.102067757036459,"altitude":-0.058068533205527816}}}}
Exactly the same as with httpget. But when i use httpget I can print for example "response.data.result.uv" to the log. With asynchttpGet trying to do the same throws:
"groovy.lang.MissingPropertyException: No such property: result for class: java.lang.String on line 105 (asyncHTTPHandler)
def params = [
uri: "https://api.openuv.io/api/v1/uv?lat=${lat}&lng=${lng}",
headers: [ 'x-access-token' : key ]
]
asynchttpGet("asyncHTTPHandler", params)
}
def asyncHTTPHandler(response, data) {
log.debug "Request was successful, $response.status"
log.debug response.data
log.debug response.data.result.uv
}
Weird since log.debug response.data returns exactly the same..