javax.crypto.AEADBadTagException: Tag mismatch! Error when encrypting String

I wrote a simple Encryption and Decryption helper class for my android app to encrypt and store Strings securely.

It consists of a single static public method to encrypt, then it calls a private static method to decrypt the encrypted message and returns it. I wrote the method this way to check if the message is intact after encryption/decryption.

I wrote a simple JUnit test with a String and called AssertEquals on the String before and after sending it to the Crypto encryption method.

I get this following errors from running the test:

javax.crypto.AEADBadTagException: Tag mismatch!

The error stack:

at com.sun.crypto.provider.GaloisCounterMode.decryptFinal(GaloisCounterMode.java:571)
at com.sun.crypto.provider.CipherCore.finalNoPadding(CipherCore.java:1046)
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:983)
at com.sun.crypto.provider.CipherCore.doFinal(CipherCore.java:845)
at com.sun.crypto.provider.AESCipher.engineDoFinal(AESCipher.java:446)
at javax.crypto.Cipher.doFinal(Cipher.java:2165)
at util.Crypto.decrypt(Crypto.java:94)
at util.Crypto.encrypt(Crypto.java:64)
at com.example.ali.meappley.CryptoTest.encryptAndDecryptTest(CryptoTest.java:29)

I'm new to cryptography, but I read different stackoverflow replies and couldn't find anything of help. Some users suggested calling cipher.update(someByteArray) before calling cipher.doFinal(someByteArray) but I couldnt manage to get it working. Any suggestions?

This is my helper class

public class Crypto {

//public methods

//public static encrypt method
public static String encrypt(String messageToEncrypt, @Nullable byte[] associatedData) throws NoSuchPaddingException,
        NoSuchAlgorithmException,
        InvalidAlgorithmParameterException,
        InvalidKeyException,
        BadPaddingException,
        IllegalBlockSizeException {

    byte[] plainBytes = messageToEncrypt.getBytes();
/////////////////////////////////////////////////////////////////
    SecureRandom secureRandom = new SecureRandom();
    byte[] key = new byte[16];
    secureRandom.nextBytes(key);
    SecretKey secretKey = new SecretKeySpec(key, "AES");

    byte[] iv = new byte[12]; //NEVER REUSE THIS IV WITH SAME KEY
    secureRandom.nextBytes(iv);

    final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
    GCMParameterSpec parameterSpec = new GCMParameterSpec(128, iv); //128 bit auth tag length
    cipher.init(Cipher.ENCRYPT_MODE, secretKey, parameterSpec);

    if (associatedData != null) {
        cipher.updateAAD(associatedData);
    }

    byte[] cipherText = cipher.doFinal(plainBytes);

    ByteBuffer byteBuffer = ByteBuffer.allocate(4 + iv.length + cipherText.length);
    byteBuffer.putInt(iv.length);
    byteBuffer.put(iv);
    byteBuffer.put(cipherText);
    byte[] cipherMessage = byteBuffer.array();

    Arrays.fill(key,(byte) 0); //overwrite the content of key with zeros
    ///////////////////////////////////////////////////////////////////

    byte[] decrypted = decrypt(cipherMessage, null, key);

    return decrypted.toString();
}

//public static decrypt method
private static byte[] decrypt(byte[] cipherMessage, @Nullable byte[] associatedData, byte[] key) throws NoSuchPaddingException,
        NoSuchAlgorithmException,
        InvalidAlgorithmParameterException,
        InvalidKeyException,
        BadPaddingException,
        IllegalBlockSizeException {

    ByteBuffer byteBuffer = ByteBuffer.wrap(cipherMessage);
    int ivLength = byteBuffer.getInt();
    if(ivLength < 12 || ivLength >= 16) { // check input parameter
        throw new IllegalArgumentException("invalid iv length");
    }
    byte[] iv = new byte[ivLength];
    byteBuffer.get(iv);
    byte[] cipherText = new byte[byteBuffer.remaining()];
    byteBuffer.get(cipherText);

    final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
    cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(key, "AES"), new GCMParameterSpec(128, iv));
    if (associatedData != null) {
        cipher.updateAAD(associatedData);
    }

    cipher.update(cipherText);
    byte[] plainText= cipher.doFinal(cipherText);

    return plainText;
}