java.lang.NullPointerException: signingKey cannot be null

Issue

The following error is observed on SAML SP protected by RH-SSO SAML adapter:

Caused by: java.lang.NullPointerException: signingKey cannot be null
    at javax.xml.crypto.dsig.dom.DOMSignContext.<init>(DOMSignContext.java:100)
    at org.keycloak.saml.processing.core.util.XMLSignatureUtil.sign(XMLSignatureUtil.java:412)
    at org.keycloak.saml.processing.api.saml.v2.sig.SAML2Signature.sign(SAML2Signature.java:142)
    at org.keycloak.saml.processing.api.saml.v2.sig.SAML2Signature.signSAMLDocument(SAML2Signature.java:159)
    at org.keycloak.saml.BaseSAML2BindingBuilder.signDocument(BaseSAML2BindingBuilder.java:277)
    at org.keycloak.saml.BaseSAML2BindingBuilder$BasePostBindingBuilder.<init>(BaseSAML2BindingBuilder.java:154)
    at org.keycloak.saml.BaseSAML2BindingBuilder.postBinding(BaseSAML2BindingBuilder.java:219)
    at org.keycloak.adapters.saml.SamlUtil.sendSaml(SamlUtil.java:45)
    at org.keycloak.adapters.saml.profile.AbstractSamlAuthenticationHandler$3.sendAuthnRequest(AbstractSamlAuthenticationHandler.java:622)
    at org.keycloak.adapters.saml.AbstractInitiateLogin.challenge(AbstractInitiateLogin.java:61)
    ... 42 more