How to mock keystore class and assign mock behavior to its methods?

I have the below method which I need to write unit tests for. But I cannot spy the class KeyStore. It throws the below exception.

org.mockito.exceptions.base.MockitoException: Unable to create mock instance of type 'KeyStore'

I can mock it though. But when I go to assign the behavior for the mock methods it throws exceptions. Methods I called and the exceptions I got are as follows.

when(keyStoreMock.getCertificate(anyString())).thenReturn(certificateMock);

 java.security.KeyStoreException: Uninitialized keystore

doNothing().when(keyStoreMock).load(any(InputStream.class),Mockito.any(char[].class));

 java.lang.NullPointerException

Below is the method I'm trying to test.

     public boolean verifySignature(String filePath, String extractContentsPath, String csvParams)
                throws ServiceSDKException {
            boolean result = false;
            String typeOfCertificateStore = "";
            String certificateStoreProvider = "";
            String certificateName = "";
            SignerInformationVerifier verifier = null;
            if (filePath != null && extractContentsPath != null && csvParams != null && !filePath.isEmpty()
                    && !extractContentsPath.isEmpty() && !csvParams.isEmpty()) {
                try {
                    String[] receivedParams = csvParams.split(",");
                    typeOfCertificateStore = receivedParams[0];
                    certificateStoreProvider = receivedParams[1];
                    certificateName = receivedParams[2];
                } catch (ArrayIndexOutOfBoundsException e) {
                    throw new ServiceSDKException("csvParams should have type of certificate store, certificate store provider and certificate name respectively", e);
                }
                try {
                    Path signedDataFilePath = Paths.get(filePath);
                    Path pathToExtractContents = Paths.get(extractContentsPath);

                    KeyStore msCertStore = KeyStore.getInstance(typeOfCertificateStore, certificateStoreProvider);
                    msCertStore.load(null, null);
                    try {
                        verifier = new JcaSimpleSignerInfoVerifierBuilder()
                                .setProvider(certificateStoreProvider)
                                .build(((X509Certificate) msCertStore.getCertificate(certificateName)));
                    } catch (Exception e) {
                        throw new ServiceSDKException("Exception occurred when building certificate",e);
                    }
                    verify(signedDataFilePath, pathToExtractContents, verifier);
                    result = true;
                } catch (KeyStoreException | NoSuchProviderException | IOException | NoSuchAlgorithmException
                        | CertificateException e) {
                    result = false;
                    throw new ServiceSDKException("Exception occurred while preparing to verify signature " , e);
                }
            } else {
                throw new ServiceSDKException("FilePath,extract contents path or csv params cannot be empty or null");
            }
            return result;
        }

How can I mock KeyStore and its method behaviors? Please advice.