```python
import uuid
from collections import deque
from typing import Dict, Any, Optional

class JobQueue:
    """
    A basic FIFO job queue implementation.
    """
    def __init__(self):
        # Queue of jobs waiting to be processed (stores job_id)
        self._pending_queue: deque[str] = deque()
        
        # Dictionary to store job status and results (key: job_id)
        # Value format: {'status': str, 'data': dict}
        self._job_store: Dict[str, Dict[str, Any]] = {}

    def add_job(self, job_id: str, data: Dict[str, Any]) -> str:
        """
        Adds a job to the queue.

        Args:
            job_id: Unique identifier for the job.
            data: The data payload for the job.

        Returns:
            The job_id.
        """
        if job_id in self._job_store:
            raise ValueError(f"Job ID '{job_id}' already exists.")

        self._job_store[job_id] = {
            'status': 'PENDING',
            'data': data
        }
        self._pending_queue.append(job_id)
        return job_id

    def process_next_job(self) -> Optional[str]:
        """
        Simulates processing the next job in the queue. 
        This method should be called externally to simulate worker activity.

        Returns:
            The job_id that was processed, or None if the queue is empty.
        """
        if not self._pending_queue:
            return None

        job_id = self._pending_queue.popleft()
        
        # Simulate processing
        self._job_store[job_id]['status'] = 'PROCESSING'
        
        try:
            # --- Job Execution Simulation ---
            # In a real system, this would be where the worker executes the task.
            # We simulate a successful execution based on the input data.
            data = self._job_store[job_id]['data']
            task = data.get('task', 'unknown')
            
            if task == "process_data":
                result = {"status": "SUCCESS", "processed_data_count": 100, "input_task": task}
            else:
                result = {"status": "ERROR", "message": f"Unknown task: {task}"}

            self._job_store[job_id]['status'] = 'COMPLETED'
            self._job_store[job_id]['result'] = result
            return job_id
        except Exception as e:
            self._job_store[job_id]['status'] = 'FAILED'
            self._job_store[job_id]['error'] = str(e)
            return job_id


    def get_result(self, job_id: str) -> Optional[Dict[str, Any]]:
        """
        Retrieves the result of a completed job.

        Args:
            job_id: The ID of the job.

        Returns:
            The result dictionary if the job is completed, otherwise None.
        """
        job = self._job_store.get(job_id)
        
        if job is None:
            return None
        
        if job['status'] == 'COMPLETED':
            return job.get('result')
        
        # Return None if the job is not completed yet
        return None

# Example Usage Demonstration (Optional, but useful for testing)
if __name__ == '__main__':
    queue = JobQueue()
    print("--- Adding Jobs ---")
    job_id1 = queue.add_job("job1", {"task": "process_data", "payload": 1})
    print(f"Added job 1: {job_id1}")
    
    job_id2 = queue.add_job("job2", {"task": "process_data", "payload": 2})
    print(f"Added job 2: {job_id2}")

    print("\n--- Checking Results (Before Processing) ---")
    print(f"Result for job1: {queue.get_result(job_id1)}") # Should be None

    print("\n--- Processing Jobs (Simulating Worker) ---")
    
    # Process job 1
    queue.process_next_job()
    print(f"Processed {job_id1}")

    # Process job 2
    queue.process_next_job()
    print(f"Processed {job_id2}")
    
    print("\n--- Checking Results (After Processing) ---")
    
    # Check job 1 result
    result1 = queue.get_result(job_id1)
    print(f"Result for job1: {result1}") 

    # Check job 2 result
    result2 = queue.get_result(job_id2)
    print(f"Result for job2: {result2}") 
    
    # Check nonexistent job
    result_none = queue.get_result("nonexistent")
    print(f"Result for nonexistent job: {result_none}")
```