React Native: Possible unhandled promise rejection

catch function in your api should either return some data which could be handled by Api call in React class or throw new error which should be caught using a catch function in your React class code. Latter approach should be something like:

return fetch(url)
.then(function(response){
  return response.json();
})
.then(function(json){
  return {
    city: json.name,
    temperature: kelvinToF(json.main.temp),
    description: _.capitalize(json.weather[0].description)
  }
})
.catch(function(error) {
console.log('There has been a problem with your fetch operation: ' + error.message);
 // ADD THIS THROW error
  throw error;
});

Then in your React Class:

Api(region.latitude, region.longitude)
  .then((data) => {
    console.log(data);
    this.setState(data);
  }).catch((error)=>{
     console.log("Api call error");
     alert(error.message);
  });