Facing issue in my batch test class. I am inserting simple Account and Opportunity in my test class. If we set(seealldata = false) the class fails in the sandbox and getting below error
System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, EventbriteSync.EventbriteOpportunityTrigger: execution of AfterInsert caused by: System.NullPointerException: Attempt to de-reference a null object Trigger.EventbriteSync.EventbriteOpportunityTrigger: line 4, column 1: 
Facing the issue in 'Eventbrite OpportunityTrigger' which is from the managed package and we can't see the code.
If I set (seealldata = true) in test passes in the sandbox but we will get below when moving to Production
System.UnexpectedException: No more than one executeBatch can be called from within a test method. Please make sure the iterable returned from your start method matches the batch size, resulting in one executeBatch invocation. Stack Trace: External entry point
There any way to fix this? Thanks in advance.