Exception (28) Arduino/ESP8266

My ESP Module gets connected to the wifi but its is not connecting to the Firebase. in my serial monitor, I'm having this -

connected: 192.168.31.5

Exception (28): epc1=0x4021509e epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000

stack>>>

ctx: cont sp: 3ffffda0 end: 3fffffc0 offset: 01a0 3fffff40: 3ffeff28 4020fb10 3fffff8c 40205a6e
3fffff50: 00000100 3ffeec08 00000000 3ffeed58
3fffff60: 3fffdad0 00000000 3ffeeba4 40205b9e
3fffff70: 3fffdad0 00000000 3ffeeba4 40201126
3fffff80: 00000000 00000000 ffefeffe 00006e6f
3fffff90: 00000000 00efeffe feefeffe feefeffe
3fffffa0: feefeffe 00000000 3ffeed28 40206a80
3fffffb0: feefeffe feefeffe 3ffe8578 4010054d
<<

Here is my code sample.

#include <ESP8266WiFi.h>
#include <FirebaseArduino.h> 

#define FIREBASE_HOST "confidential.firebaseio.com"
#define FIREBASE_AUTH "confidential..........."
#define WIFI_SSID "Open Wifi"
#define WIFI_PASSWORD "bolajabena"

String status;
void setup() {
Serial.begin(115200);
pinMode(4, OUTPUT);
// connect to wifi.
WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
Serial.print("connecting");
while (WiFi.status() != WL_CONNECTED) {
Serial.print(".");
delay(500);
}
Serial.println();
Serial.print("connected: ");
Serial.println(WiFi.localIP());
Firebase.begin(FIREBASE_HOST, FIREBASE_AUTH);
}
void loop() {
// get value

status = Firebase.getString("on");
if(status=="1"){
digitalWrite(4,HIGH);
Serial.println("LED ON");
}
else {
Serial.println("LED OFF");
digitalWrite(4,LOW);  
return;
}
}

how can I overcome this problem? I'm very new in Arduino