503 Service Unavailable error

[alchemistgo87]

I uploaded code to app engine and tried calling apis using api explorer. Its always showing this error:

503 Service Unavailable

Show headers -

{
"error": {
"errors": [
{
"domain": "global",
"reason": "backendError",
"message": "java.lang.NullPointerException"
}
],
"code": 503,
"message": "java.lang.NullPointerException"
}
}