Hello,
I am trying to create an event in Emma from Salesforce Apex class which will get invoked from the process builder. However, I am geting the following error:
​​​​​​An Apex error occurred: System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out.
Here is my sample apex class. There is no DML operation here.
public with Sharing class CreateEmmaEvent{
    @invocableMethod(label='Create Emma Event on RFI submission')
    public static void CreateEmmaEventForRFI(List<Id> RFIIds){
        List<RFI_Transaction__c> rfis = [Select ID, Contact__c, Lead__c, RFI_Email__c from RFI_Transaction__c where ID in :RFIIds];
        for (RFI_Transaction__c rfi : rfis){
           JSONGenerator gen = JSON.createGenerator(true);    
            gen.writeStartObject();      
            gen.writeStringField('event_name', 'rfi_submitted');
            gen.writeStringField('email',rfi.RFI_Email__c);
            gen.writeEndObject();
            String jsonS = gen.getAsString();
            System.debug('jsonMaterials'+jsonS);
            
            HttpRequest req = new HttpRequest();
            req.setMethod('POST');
            req.setbody(jsonS);
            req.setEndpoint('callout:emma_event/v1/1923372/events');
            req.setHeader('APIKEY', '{!$Credential.Password}');
            HttpResponse res = new Http().send(req);
           
        }
    }   
}
By the way, I created a Named Credential in Salesforce with the endpoint https://events.e2ma.net/ and the public api key as the password.
I would greatly appreciate if someone can guide me in the right direction to get this working. 

Thanks,
Geetha