My application continues crashing and I don't know why

Question

I'm trying to write a messaging application, but when I start it, it crashes, and I do not get an error message, so I have no clue why.

Launch Activity:

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.widget.*;

public class ConnectActivity extends AppCompatActivity {

    private Client client;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        final EditText txtServerAddress = (EditText) findViewById(R.id.edit_text_server_address);

        final EditText txtPortNumber = (EditText) findViewById(R.id.edit_text_port_number);

        final EditText txtUsername = (EditText) findViewById(R.id.edit_text_username);

        final Button btnLogIn = (Button) findViewById(R.id.button_log_in);
        btnLogIn.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {
                String username = txtUsername.getText().toString();
                if(username.length() == 0)
                    return;

                String portNumber = txtPortNumber.getText().toString();
                if(portNumber.length() == 0)
                    return;

                String serverAddress = txtServerAddress.getText().toString();
                if(serverAddress.length() == 0)
                    return;

                int port = 0;

                try
                {
                    port = Integer.parseInt(portNumber);
                }
                catch(Exception en)
                {
                    return;
                }

                ClientActivity clientActivity = new ClientActivity(client);
                client = new Client(serverAddress, port, username, clientActivity);


                if(!client.start())
                    return;

                Intent intent = new Intent(ConnectActivity.this, ClientActivity.class);
                startActivity(intent);
            }
        });

        setContentView(R.layout.activity_connect);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
    }
}

Launch Content xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.liftedstarfish.lifte.gpschat0_2.ConnectActivity"
    tools:showIn="@layout/activity_connect">

    <TextView
        android:id="@+id/text_view_server_address"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/server_address"
        android:textColor="@android:color/black"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/text_view_port_number"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/port_number"
        android:textColor="@android:color/black"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/edit_text_server_address" />

    <TextView
        android:id="@+id/text_view_username"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="@string/username"
        android:textColor="@android:color/black"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/edit_text_port_number" />

    <EditText
        android:id="@+id/edit_text_server_address"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPersonName|numberDecimal"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/text_view_server_address" />

    <EditText
        android:id="@+id/edit_text_port_number"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="number"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/text_view_port_number" />

    <EditText
        android:id="@+id/edit_text_username"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:ems="10"
        android:inputType="textPersonName"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/text_view_username" />

    <Button
        android:id="@+id/button_log_in"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:text="@string/log_in"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintRight_toRightOf="parent" />
</android.support.constraint.ConstraintLayout>

Launch Activity xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.liftedstarfish.lifte.gpschat0_2.ConnectActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/content_connect" />

</android.support.design.widget.CoordinatorLayout>

Android Manifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.liftedstarfish.lifte.gpschat0_2">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".ClientActivity"
            android:label="@string/title_activity_client"
            android:theme="@style/AppTheme.NoActionBar" />
        <activity
            android:name=".ConnectActivity"
            android:label="@string/title_activity_connect"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

As far as I can tell, I've done everything correctly, but the application is still crashes. I've tried cleaning the application and rebuilding it, but that did not help.