'Case2' doesn;t doesn't live in classpath for javac to construct a Element. The stacktrace shows a code path as JavacCompiler Code.
I am not sure which part it started passing a 'Null' value to next layers. This could be a JavacCompiler API. I am just guessing.

May be 'ElementNotFoundException' was required.

bash-3.00$ cat CaseAP.java
import java.util.*;
import javax.annotation.processing.*;
import javax.lang.model.element.*;
import javax.lang.model.type.*;
import javax.lang.model.util.*;
import static javax.lang.model.SourceVersion.*;
import static javax.lang.model.type.TypeKind.*;

@SupportedAnnotationTypes("*")
@SupportedSourceVersion(RELEASE_6)
@SupportedOptions("-verbose")
public class CaseAP  extends  AbstractProcessor {
    public void init(ProcessingEnvironment penv) {
        super.init(penv);
    }
    public boolean process(Set<? extends TypeElement> typeElementSet,
            RoundEnvironment renv) {
        Elements elementUtils = processingEnv.getElementUtils();
        TypeElement case2 = elementUtils.getTypeElement("Case2");
	// Case2 class doesn;t exisit in classpath to construct a Element.
        if ( case2 != null ) {
            System.out.println(" element received as "+case2.toString());
        }
        return true ;
    }
}
bash-3.00$ cat Case1.java
public class Case1 {
}
bash-3.00$ $JAVA_HOME/bin/javac -classpath $CLASSPATH -processor CaseAP Case1.java


An annotation processor threw an uncaught exception.
Consult the following stack trace for details.
java.lang.NullPointerException
        at com.sun.tools.javac.main.JavaCompiler.resolveIdent(JavaCompiler.java:497)
        at com.sun.tools.javac.model.JavacElements.nameToSymbol(JavacElements.java:151)
        at com.sun.tools.javac.model.JavacElements.getTypeElement(JavacElements.java:134)
        at com.sun.tools.javac.model.JavacElements.getTypeElement(JavacElements.java:46)
        at CaseAP.process(CaseAP.java:19)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor(JavacProcessingEnvironment.java:497)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.discovery(JavacProcessingEnvironment.java:421)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(JavacProcessingEnvironment.java:569)
        at com.sun.tools.javac.main.JavaCompiler.processAnnotations(JavaCompiler.java:843)
        at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:629)
        at com.sun.tools.javac.main.Main.compile(Main.java:756)
        at com.sun.tools.javac.main.Main.compile(Main.java:681)
        at com.sun.tools.javac.main.Main.compile(Main.java:670)
        at com.sun.tools.javac.Main.compile(Main.java:70)
        at com.sun.tools.javac.Main.main(Main.java:55)

bash-3.00$$JAVA_HOME/bin/java -version
java version "1.6.0-auto"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-auto-292b)
Java HotSpot(TM) Server VM (build 1.6.0-ea-b56, mixed mode)