How to solve java.lang.NullPointerException error?

When I run my Java program, it gives me an error on this line

compiler.getTask(null, null, new DiagnosticCollector(), null, null, compilationUnits);

Error I am getting is:

Exception in thread "main" java.lang.NullPointerException

    at AnotherClassLoader.loadClass(test.java:58)
    at test.main(test.java:30)
    at Main.main(Main.java:68)

Can you please tell me how can I solve this error?