An unhandled exception occured: GetStringAsync()

Just getting started with Xamarin... created a Xamarin Forms cross-platform app.

I am trying to fetch data from a REST API and place the data into a ListView.

I know the API url is correct as I've tested it separately in the browser.

The line of code which causes the exception is the "GetStringAsync()" call:

public async void GetData() {
_url = "http://jsonplaceholder.typicode.com/posts/1";

    var content = await _client.GetStringAsync(_url);  //  <---- HERE

    var items = JsonConvert.DeserializeObject<List<Post>>(content);

    _posts = new ObservableCollection<Post>(items);

    listView.ItemsSource = _posts;

}

public class Post {
public int Id { get; set; }
public string Title { get; set; }
public string Body { get; set; }
}

<?xml version="1.0" encoding="utf-8" ?>

<ContentPage.Content>

        <ListView x:Name="listView">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <TextCell Text="{Binding Title}" />
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackLayout>
</ContentPage.Content>

The program shows a message on opening in a label. Then upon clicking a button, I want to retrieve the data and show it on the same page, in the ListView (which contains nothing prior).

Running the program results in the error:

"An unhandled exception occured."

Things I've tried...

1. I've set the "advanced settings" in the Android project under "properties | android options | advanced". I've tried all combinations for the two settings of "httpclient" and "ssl/tls" implementation, despite my API url not being secure (https - I've tried both ways).

2. I've removed the GetStringAsync call and hard-coded a json object and it loaded fine into the Listview.

3. I've wrapped the call in a try/catch and changed the regular call to the method from "GetData();" in the click event method to:

   Task.Run(async () => await GetData());

and while the error goes away, nothing happens data-wise either.

3. I've Googled the @#$% out of the error and circumstances and cannot seem to find an answer.

Any ideas out there?