Nullpointerexception on com.android.okhttp.internal.http.HttpStream.writeRequestHeaders

FATAL EXCEPTION: main Process: nixerton.everemotequicktask, PID: 11611 java.lang.NullPointerException: Attempt to invoke interface method 'void com.android.okhttp.internal.http.HttpStream.writeRequestHeaders(com.android.okhttp.Request)' on a null object reference at com.android.okhttp.internal.http.HttpEngine.readResponse(HttpEngine.java:593) at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:471) at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getResponse(HttpURLConnectionImpl.java:407) at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getResponseMessage(HttpURLConnectionImpl.java:534) at com.android.tools.profiler.support.network.httpurl.TrackedHttpURLConnection.trackResponse(TrackedHttpURLConnection.java:86) at com.android.tools.profiler.support.network.httpurl.TrackedHttpURLConnection.tryTrackResponse(TrackedHttpURLConnection.java:101) at com.android.tools.profiler.support.network.httpurl.TrackedHttpURLConnection.getResponseCode(TrackedHttpURLConnection.java:308) at com.android.tools.profiler.support.network.httpurl.HttpURLConnection$.getResponseCode(HttpURLConnection$.java:92) at nixerton.everemotequicktask.EveAIORemote.PostRequest(EveAIORemote.java:30) at nixerton.everemotequicktask.EveAIORemote$1.onClick(EveAIORemote.java:54) at android.view.View.performClick(View.java:6294) at android.view.View$PerformClick.run(View.java:24770) at android.os.Handler.handleCallback(Handler.java:790) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6494) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

I'm getting this Exception once my PostRequest is triggered by the button click. No idea of why this is occuring though. This is my code:

public class EveAIORemote extends AppCompatActivity {

TextView tv;
Button btn;

public void PostRequest(String url_from_text) throws IOException {
    String url = url_from_text;
    URL obj = new URL(url);
    HttpURLConnection con = (HttpURLConnection) obj.openConnection();
    con.setRequestMethod("GET");
    con.setRequestProperty("User-Agent", "Mozilla/5.0");
    int responseCode = con.getResponseCode();
    BufferedReader in = new BufferedReader(
            new InputStreamReader(con.getInputStream()));
    String inputLine;
    StringBuffer response = new StringBuffer();
    while ((inputLine = in.readLine()) != null) {
        response.append(inputLine);
    }
    in.close();
    //print in String
    System.out.println(response.toString());
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_eve_aioremote);
    btn = (Button) findViewById(R.id.Button1);
    tv = (TextView) findViewById(R.id.textView);

    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            try {
                PostRequest("apilink");
            } catch (IOException e){
                e.printStackTrace();
            }
        }
    });
}
}

I didn't put my actual api link there as it contains several private informations, but I get the same error with other similar api links.