How to fix Null Pointer Exception when Processing reads from Arduino - nullpointerexception

Question

I am trying to construct an automatic flow rack system to replace a manual Kanban system. I works by using an Arduino with an ultrasonic sensor attached to back of a rack. The sensor reads the distance from the last box in the rack to it.
the idea behind this is that when someone removes a box from the rack, the other boxes slide forward and the distance from the last box to the back of the rack gets larger. The amount of distance triggers a signal to the restocker. The signal is green if the rack is full, yellow if there are still a few boxes left, and red if there is one or no boxes left.
The Arduino code is working fine and serial prints "Green", "Yellow", or "Red" depending on distance.
Where I am having issues is translating that readout into a visual system. I want a display of the rack on the computer screen with each shelf having a color to show how stocked it is. I'm using Processing to make this and I've had success getting it to print the output instead of Arduino, however when I make an "if" statement to check if the string being read from the Arduino matches a predetermined string, I get a null pointer exception and am not sure how to fix it.
I need the if statement so that it will show green box on that shelf if the serial print reads "Green", and the same for yellow and red.
import processing.serial.*;
Serial myPort; //Create object from serial class
String val;
String str1;
void setup()
{
str1 = "Green";
String portName = Serial.list()[1];
myPort = new Serial(this, portName, 115200);
size(500, 500);
line(5, 5, 5, 495); //these lines construct the flow rack diagram
line(5, 5, 495, 5);
line(495, 5, 495, 495);
line(5, 495, 495, 495);
line(250, 5, 250, 495);
line(5, 250, 495, 250);
}
void draw()
{
if (myPort.available()>0)
{
val = myPort.readStringUntil('\n');
}
if(val.equals(str1) == true){ //checks if val is Green
println(str1);
}
}

Answer 1

NullPointerExceptions are usually easy to fix. All you have to do is check if the variable your are trying to access is null before accessing it or you could initialize your variables on top :
//check if var is null trick
void draw()
{
if (myPort.available()>0)
{
val = myPort.readStringUntil('\n');
}
if (val != null)
{
if(val.equals(str1) == true){ //checks if val is Green
println(str1);
}
}
}
// initializing trick
Serial myPort = new Serial();
String val = "";
String str1 = "";
Btw should should do the same if (... != null) trick for you first if statement since myport could be null too!
I think your problem is not the NullPointerException but the fact that your readStringUntill function returns null. You should look into that function and make sure it works correctly (and that you give it the right parameter).