[SalesForce] Having trouble writing Apex trigger on custom object to update a lookup fields

I have a trigger that maps information from one Custom Object (Agents) to another Custom Object (Trident Contracts). I am able to update Seller Agent Name in the Trident Contract Mapping with the ID. see code below

trigger SellAgent on Trident_Contract__c (before insert, before update) {
    Set <String> AgentID = new Set <String>();
    for (Trident_Contract__c tc : trigger.new) {
        if (tc.Seller_Agent_ID__c != NULL) {
            AgentID.add(tc.Seller_Agent_ID__c);
        }
    }
    Map<String, Agent__c> Agent = new map <String, Agent__C>();
    for (Agent__c obj : [SELECT ID, Agent_ID__c,
                         from Agent__c
                         Where Agent_ID__c
                         IN: AgentID]) {
        Agent.put(obj.Agent_ID__c, obj);
    }
    for (Trident_Contract__c tc : trigger.new) {
        if (Agent.containsKey(tc.Seller_Agent_ID__c)) tc.Seller_Agent_Lookup__c = Agent.get(tc.Seller_Agent_ID__c).ID;
    }
}

Now I want to add to this trigger and update another field on the Trident Contract (Seller Agent Office)using a map from the Agent (office) field. But I get this error message when I run the test

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, AgentInfo: execution of BeforeInsert
caused by: System.NullPointerException: Attempt to de – reference a null object
Trigger.AgentInfo: line 20, column 1: []

Here is my updated code and Test. Please let me know what I am doing wrong.

trigger SellAgent on Trident_Contract__c (before insert, before update) {
    Set <String> AgentID = new Set <String>();
    for (Trident_Contract__c tc : trigger.new) {
        if (tc.Seller_Agent_ID__c != NULL) {
            AgentID.add(tc.Seller_Agent_ID__c);
        }
    }
    Map<String, Agent__c> Agent = new map <String, Agent__c>();
    for (Agent__c obj : [SELECT ID, Agent_ID__c, Agent_Office__c
                         from Agent__c
                         Where Agent_ID__c
                         IN: AgentID]) {
        Agent.put(obj.Agent_ID__c, obj);
    }
    for (Trident_Contract__c tc : trigger.new) {
        if (Agent.containsKey(tc.Seller_Agent_ID__c)) tc.Seller_Agent_Lookup__c = Agent.get(tc.Seller_Agent_ID__c).ID;
        tc.Seller_Agent_Office__c = Agent.get(tc.Seller_Agent_ID__c).Agent_Office__c;
    }
}

@isTest
private class TridentContract {
    static testmethod void testcontracts() {
        Trident_Contract__c contract1 = new Trident_Contract__c(Name = '1234568723423', Buyer_Agent_ID__c = '1234', Buyer_Agent_Branch__c = 'a02q0000002rils', Seller_Agent_ID__c = '1234', Seller_Agent_Branch__c = 'a02q0000002rils');

        test.startTest();

        insert contract1;


    }
}

Thank you for any suggestions. The trigger works in the Sandbox but it will not pass the test or deploy.