I cannot figure out Uncompilable source code - variable might not have been initialized

Question

I'm new to learning Java and am writing a BMI Calculator program. I've fixed so many errors in my program but just cannot figure this one out! Hopefully, you can help and it will be the last error I need to fix so it will finally execute without errors.

Then I can make some changes for it to do exactly what I want such as exception handling to deal with incorrect values entered etc.

I've spent so much time on this and have edited it so much that I have got myself so confused. All feedback welcome, I'm here to learn.

This is the error I am getting on Netbeans:

Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - variable bmi might not have been initialized at BmiCalc.main(BmiCalc.java:9)

Here is my code:

{
    double bmi;
    if (bmi < 18.5) {
        System.out.println("You are underweight");
    }
    else if (bmi >= 18.5 && bmi <= 24.9) {        
        System.out.println("You are a healthy, normal weight");
    }