MainActivity not launching when it is run

Question

I am developing a simple login and register android app and everything have been coded and no errors were issued. The app can be ran successfully and all, but, the screen doesn't show the launcher activity. Like, it only shows up and then immediately close the app. I hope someone can catch what I am trying to say here. Also, I'm 100% sure that i have registered the activities on the manifest file.

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.READ_PHONE_STATE"/>

<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"
        tools:ignore="GoogleAppIndexingWarning">

    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN"/>
            <category android:name="android.intent.category.LAUNCHER"/>
            <category android:name="android.intent.category.DEFAULT"/>
        </intent-filter>
    </activity>

    <activity
            android:name=".LoginActivity">
    </activity>
    <activity
            android:name=".RegisterActivity">
    </activity>


    <service android:name=".MyFirebaseMessagingService">
        <intent-filter>
            <action android:name="com.google.firebase.MESSAGING_EVENT"/>
        </intent-filter>
    </service>
    <service android:name=".MyFirebaseInstanceIDService">
        <intent-filter>
            <action android:name="com.google.firebase.INSTANCE_ID_EVENT"/>
        </intent-filter>
    </service>

    <meta-data
            android:name="android.app.alertbox"
            android:resource="@xml/automotive_app_desc"/>
</application>

</manifest>

And this is what i get from the error log.

Caused by: kotlin.KotlinNullPointerException
        at com.example.acer.alertbox.MainActivity.onCreate(MainActivity.kt:17)
        at android.app.Activity.performCreate(Activity.java:6910)
        at 
   android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
        at 
   android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2746)
        at 
   android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2864) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1567) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:156) 
        at android.app.ActivityThread.main(ActivityThread.java:6523) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at 


 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:941) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:831)

This is my MainActivity.kt code:

package com.example.acer.alertbox

import android.content.Intent
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val btnLogin: Button? = null
        val btnRegister: Button? = null

        btnLogin!!.setOnClickListener {
            val intent = Intent(this@MainActivity, LoginActivity::class.java)
            startActivity(intent)
        }

        btnRegister!!.setOnClickListener {
            val regIntent = Intent(this@MainActivity, RegisterActivity::class.java)
            startActivity(regIntent)
        }

    }
}

*Update: I have solved the issues by removing the null pointers. I did not know that there was any error because I did not check nor did I know how to open the Log Error so that is why I did not know this type of question has been asked before. I'm a new android developer by the way.

can you post your crash log. what i believe your app is crashing some where — Abdul Waheed, Dec 05 '18 at 06:29
Please post relevant activity code, and a copy of the logcat if the app is crashing rather than just closing itself. — TheGamer007, Dec 05 '18 at 06:38
Please, Can you show me your Error log or your Manifest File? — Prince Dholakiya, Dec 05 '18 at 06:43
the error log says Session 'app': Error Launching Activity @MadLeo — programmer211216, Dec 05 '18 at 06:58
post your error log fully can't help with Error Launching Activity — MadLeo, Dec 05 '18 at 07:00
@AbdulWaheed the log says Session 'app':Error Launching Activity — programmer211216, Dec 05 '18 at 07:00
@MadLeo i'm sorry but where can i get the error log though? I am confused with the event log at the bottom — programmer211216, Dec 05 '18 at 07:11
In android studio at bottom left you can find Logcat click on that, there are log level filter also available, you can filter by error — MadLeo, Dec 05 '18 at 07:33
event log is not required search Logcat in android studio bottom left corner — MadLeo, Dec 05 '18 at 07:42
@MadLeo just posted the error logs (some) since it is too long and i dont know which part should i have copied. — programmer211216, Dec 05 '18 at 07:46
the issue is kotlin.KotlinNullPointerException in your MainActivity search for that where its happening — MadLeo, Dec 05 '18 at 07:50
I have just posted the MainActivity code above. i deleted the "!!" symbol but it is underlined red @MadLeo — programmer211216, Dec 05 '18 at 07:56
alright I have changed it by removing the null. Problem solved. Thanks to everyone @MadLeo — programmer211216, Dec 05 '18 at 08:08

score 0 · Answer 1 · answered Dec 05 '18 at 06:39

0

Do you Add this ⬇ in your Activity tag within manifest file.

            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>

This code is use for first launcher activity specification.

answered Dec 05 '18 at 06:39

Prince Dholakiya

3,017
23
38

yes i did. you can refer to the above manifest code. i've just posted it. – programmer211216 Dec 05 '18 at 06:59

score 0 · Accepted Answer · answered Dec 05 '18 at 08:04

So After a long discussion i can see that you are setting null to your button

val btnLogin: Button? = null
val btnRegister: Button? = null

and then on null you are setting onClickListener that's why it's crashing so you have to set the reference of button something like this after your declaration

btnLogin = findViewById(R.id.buttonid) as Button
btnRegister = findViewById(R.id.buttonid) as Button

the button id you can find from your activity_main layout file.

yes I have just changed even before i saw answer. Thanks for clues. — programmer211216, Dec 05 '18 at 08:10

MainActivity not launching when it is run

2 Answers2