Here is my Code.   The error occurs when my users convert a lead without creating an opportuntity.


trigger checkPrimaryCampaignSource on Lead (after update) {
    //This will be a set of all opportunity IDs for leads that were just converted
    Set<Id> convertedOppIds = new Set<Id>();
    for (Lead l :Trigger.new) {
        if (l.IsConverted == true) {
            convertedOppIds.add(l.ConvertedOpportunityId);
        }
    }
  
    //Now get a set of all of the campaign members related to the converted leads who have not responded to any of the campaigns
    List<CampaignMember> campaignMembers = [SELECT Id, Lead.ConvertedOpportunityId FROM CampaignMember WHERE Lead.ConvertedOpportunityId IN :convertedOppIds AND HasResponded = false];
    //Construct a set of opportunity ids that were converted from leads who did not respond to the campaign
    Set<Id> nonResponsiveOppIds = new Set<Id>();
    for (CampaignMember cm : campaignMembers) {
        nonResponsiveOppIds.add(cm.Lead.ConvertedOpportunityId);
    }
  
    //Now we can get a list of all of the opportunities where we need to remove the primary campaign source
    List<Opportunity> oppsToUpdate = [SELECT CampaignId FROM Opportunity WHERE Id IN :nonResponsiveOppIds];
    for (Opportunity opp : oppsToUpdate) {
        opp.CampaignId = null;
    }
  
    if (oppsToUpdate.size() > 0) {
        update oppsToUpdate;
    }
}