problem with bytes to string

I try to convert a numer of bytes containing ascii to a string to display them on a oled display.
The bytes come from a MQTT reciever.
The string is not being null-terminated but print a few extra characters.

void callback(char* topic, byte *payload, unsigned int length) {
    //String stringOne[length];
    //Serial.println("-------new message from broker-----");

    Serial.print("channel: ");
    Serial.print(" - data: ");  
    Serial.write(payload, length);
    Serial.print(" - length: ");

    String stringOne = (char*)payload;
    stringOne += '\0';

    display.drawString(0, 10, "Instelling: "+stringOne);
    Serial.println("Instelling: "+stringOne);

the output I get is:
channel: ketel/modulationLevel - data: 0.00 - length: 4
Instelling: 0.00a122
channel: th/pv - data: 19.44 - length: 5
Instelling: 19.44ationLevel

I tried to put a \0 at the end of the string but that does not work obviously.

What am I doing wrong?

You really should post your entire sketch when asking for help.

I believe this is where you problem is:

    String stringOne = (char*)payload;
    stringOne += '\0';

When you assign the character string payload to stringOne it is copied until a null is encountered. Therefore it copies memory until it encounters a 0. Obviously payload is not null-terminated.