java.lang.ClassNotFoundException: Didn't find class "com.google.android.gms.ads.MobileAdsInitProvider"

You are getting this error because you have use multiDex but some implementation part is missing. Follow below steps to resolve the error.

1) Add "multiDexEnabled true" in defaultconfig in app-level gradle file

android {
    defaultConfig {
        ...
        minSdkVersion 21
        targetSdkVersion 28
        multiDexEnabled true
    }
    ...
}

2) If your minSdkVersion is less than 21 then add below dependency.

dependencies {
  implementation 'com.android.support:multidex:1.0.3'
}

3) Use MultiDexApplication class as Application class. There are three way to use MultiDexApplication as Application class

i) Just set MultiDexApplication class in AndroidManifest.xml file

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapp">
    <application
            android:name="android.support.multidex.MultiDexApplication" >
        ...
    </application>
</manifest>

ii) If you are already using custom application class then extent MultiDexApplication in custom application class

public class MyApplication extends MultiDexApplication { ... }

iii) If it is not possible to extend MultiDexApplication because you already extend other class and can not change it then use below method in your custom application class

public class MyApplication extends SomeOtherApplication {
  @Override
  protected void attachBaseContext(Context base) {
     super.attachBaseContext(base);
     MultiDex.install(this);
  }
}

Note: I was facing same error and just solved by extending MultiDexApplication class

I just did the 'Build' > 'Clean Project' and it works.

1. First make multiDexEnable ture in android defaultConfig

   android {
       defaultConfig {
          minSdkVersion 14
          targetSdkVersion 22
          multiDexEnabled true
       }
   }
   

2. Add this in dependency if your minSdkVersion is less then 21.

   implementation 'com.android.support:multidex:1.0.3'