AES and RSA using javax.crypto

So I am assuming that I did something wrong, and my AES and RSA encryption and decryption classes are insecure. I plan on using them for a larger project, and want to make sure I didn't completely muff them first. My questions are as follows:

1. What, if anything, makes these two classes insecure?

2. What, if anything, could be neater/better form about my code?

3. What am I not thinking to ask?

AES:

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;

/**
 * Created by Gabriel Wittes on 3/15/2016.
 * A class to encrypt and decrypt AES plaintext and ciphertext, as well as to generate AES keys.
 */
public class AES {
    /**
     * Returns a new secret AES key.
     * @return a secret AES key
     */
    public static SecretKey generateKey(){
        KeyGenerator keyGenerator = null;
        try {
            keyGenerator = KeyGenerator.getInstance("AES");
            keyGenerator.init(128);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return keyGenerator.generateKey();
    }

    /**
     * Returns an AES-encrypted byte array given a plaintext string and a secret AES key.
     * @param plaintext a plaintext string
     * @param key a secret AES key
     * @return ciphertext
     */
    public static byte[] encrypt(String plaintext, SecretKey key){
        byte[] ciphertext = null;
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            cipher.init(Cipher.ENCRYPT_MODE, key);
            ciphertext = cipher.doFinal(plaintext.getBytes());
        } catch (Exception e) {
            e.printStackTrace();
        }
        return ciphertext;
    }

    /**
     * Returns the plaintext of a given AES-encrypted string, and a secret AES key.
     * @param ciphertext an AES encrypted byte array
     * @param key a secret AES key
     * @return plaintext
     */
    public static String decrypt(byte[] ciphertext, SecretKey key){
        byte[] plaintext = null;
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            cipher.init(Cipher.DECRYPT_MODE, key);
            plaintext = cipher.doFinal(ciphertext);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return new String(plaintext);
    }
}

RSA:

import javax.crypto.Cipher;
import java.security.*;

/**
 * Created by Gabriel Wittes on 3/15/2016.
 * A class to encrypt and decrypt RSA plaintext and ciphertext, as well as to generate RSA key pairs.
 */
public class RSA {
    /**
     * Returns a new RSA key pair.
     * @return public and private RSA keys
     */
    public static KeyPair generateKeyPair(){
        KeyPairGenerator keyPairGenerator = null;
        try {
            keyPairGenerator = KeyPairGenerator.getInstance("RSA");
            keyPairGenerator.initialize(2048);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return keyPairGenerator.generateKeyPair();
    }

    /**
     * Returns an RSA-encrypted byte array given a plaintext string and a public RSA key.
     * @param plaintext a plaintext string
     * @param key a public RSA key
     * @return ciphertext
     */
    public static byte[] encrypt(String plaintext, PublicKey key){
        byte[] ciphertext = null;
        try {
            Cipher cipher = Cipher.getInstance("RSA/ECB/OASP");
            cipher.init(Cipher.ENCRYPT_MODE, key);
            ciphertext = cipher.doFinal(plaintext.getBytes());
        } catch (Exception e) {
            e.printStackTrace();
        }
        return ciphertext;
    }

    /**
     * Returns the plaintext of a given RSA-encrypted string and a private RSA key.
     * @param ciphertext an RSA-encrypted byte array
     * @param key a private RSA key
     * @return plaintext
     */
    public static String decrypt(byte[] ciphertext, PrivateKey key){
        byte[] plaintext = null;
        try {
            Cipher cipher = Cipher.getInstance("RSA/ECB/OASP");
            cipher.init(Cipher.DECRYPT_MODE, key);
            plaintext = cipher.doFinal(ciphertext);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return new String(plaintext);
    }
}