Trigger Error in salesforce

Question

When I used to create the record in Event object it is showing error like:

Error: Invalid Data.
Apex trigger General_Event_Trigger caused an unexpected exception, contact your administrator: General_Event_Trigger: execution of BeforeINsert caused by: System.NullPointerException: Attempt to de-reference a null object: Class.EventClass.updateNewCustomFieldsEvent: line 369, column 1

Trigger code:

trigger General_Event_Trigger on Event (...)
{
    if (trigger.isBefore)
    {
        if (trigger.isInsert)
        {
            EventClass.updateNewCustomFieldsEvent(trigger.new);
        }
        else if (trigger.isUpdate)
        {
            EventClass.updateNewCustomFieldsEvent(trigger.new);
        }
    }
}

Apex class:

public static void updateNewCustomFieldsEvent(List<Event> eventList)
{
    for (Event eve : eventList)
    {
        if (Administrators__c.getInstance().Update_field_Last_Modified_End_User__c)
        {
            String userIdString = eve.LastModifiedById;
            eve.Last_Modified_By_End_User__c = userIdString.subString(0, 15);
            eve.Last_Modified_Date_End_User__c = eve.LastModifiedDate;
        }
    }
}

Answer 1

I just checked lastmodified by id is null on a before insert event and this makes sense.

For example (hastily written trigger based approach, not best practice!):

trigger eventTrigger on Event (before insert) {
    system.debug(logginglevel.WARN, '^^^ event last modified: '+trigger.new[0].LastModifiedById);
    system.debug(logginglevel.WARN, '^^^ userinfo:  '+userinfo.getUserId());
}

returns:

11:38:56:001 USER_DEBUG [2]|WARN|^^^ event last modified: null

11:38:56:001 USER_DEBUG [3]|WARN|^^^ userinfo: 00510000001QaKjAAK

While the null check approach is valid you would end up with blank data, alternatively using the userinfo class would give you the current user and therefore the user creating the record.

Answer 2

You should check if variable is not null or empty

eve.Last_Modified_By_End_User__c = String.isBlank( userIdString ) ? '' : userIdString.substring(0, 15)

However, like other users reasonably suggest, on before trigger userIdString might be always empty.