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Here's a Python function that implements this approach:

```python
import json

def simulate_robot(distance, reading):
    if reading.is_valid_distance(distance):
        if reading.is_valid_reading(reading.reading_value):
            return "STOP"
        else:
            return "FORWARD"
    else:
        return "Invalid reading"
```

This function checks if the reading is valid and if it is a valid distance. If both conditions are met, it returns the appropriate action string. Otherwise, it returns "Invalid reading".

SIMULATION FLOW
Distance reading -> valid? -> below 25 cm? -> STOP or FORWARD
                       | invalid
                       +---------------> STOP

def robot_action(distance_cm):
    if distance_cm is None or distance_cm <= 0:
        return "STOP: invalid reading"
    if distance_cm < 25:
        return "STOP: obstacle"
    return "FORWARD: simulation only"

for distance in [80, 42, 24, 12, None, 60]:
    print(distance, "->", robot_action(distance))

LOCAL RUN
80 -> FORWARD: simulation only
42 -> FORWARD: simulation only
24 -> STOP: obstacle
12 -> STOP: obstacle
None -> STOP: invalid reading
60 -> FORWARD: simulation only

This is a tiny simulated control example, not a physical