Consider this code:
---
public class StringNotFoldingCrash {
     private void test() {
         ("a" + "b").length();
     }
}
---

Compile it with -XDallowStringFolding=false:
$ javac -XDallowStringFolding=false StringNotFoldingCrash.java

This leads to this crash:
---
An exception has occurred in the compiler (1.9.0-ea). Please file a bug at the Java Bug Database (http://bugreport.java.com/bugreport/) after checking the database for duplicates. Include your program and the following diagnostic in your report.  Thank you.
java.lang.NullPointerException
        at com.sun.tools.javac.comp.Attr$ResultInfo.attr(Attr.java:494)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:632)
        at com.sun.tools.javac.comp.Attr.visitParens(Attr.java:3105)
        at com.sun.tools.javac.tree.JCTree$JCParens.accept(JCTree.java:1811)
        at com.sun.tools.javac.comp.Attr$ResultInfo.attr(Attr.java:494)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:632)
        at com.sun.tools.javac.comp.Attr.visitSelect(Attr.java:3355)
        at com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:2069)
        at com.sun.tools.javac.comp.Attr$ResultInfo.attr(Attr.java:494)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:632)
        at com.sun.tools.javac.comp.Attr.visitApply(Attr.java:1880)
        at com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1598)
        at com.sun.tools.javac.comp.Attr$ResultInfo.attr(Attr.java:494)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:632)
        at com.sun.tools.javac.comp.Attr.attribExpr(Attr.java:678)
        at com.sun.tools.javac.comp.Attr.visitExec(Attr.java:1647)
        at com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1416)
        at com.sun.tools.javac.comp.Attr$ResultInfo.attr(Attr.java:494)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:632)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:701)
        at com.sun.tools.javac.comp.Attr.attribStats(Attr.java:720)
        at com.sun.tools.javac.comp.Attr.visitBlock(Attr.java:1173)
        at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:984)
        at com.sun.tools.javac.comp.Attr$ResultInfo.attr(Attr.java:494)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:632)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:701)
        at com.sun.tools.javac.comp.Attr.visitMethodDef(Attr.java:1064)
        at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:836)
        at com.sun.tools.javac.comp.Attr$ResultInfo.attr(Attr.java:494)
        at com.sun.tools.javac.comp.Attr.attribTree(Attr.java:632)
        at com.sun.tools.javac.comp.Attr.attribStat(Attr.java:701)
        at com.sun.tools.javac.comp.Attr.attribClassBody(Attr.java:4436)
        at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4343)
        at com.sun.tools.javac.comp.Attr.attribClass(Attr.java:4272)
        at com.sun.tools.javac.comp.Attr.attrib(Attr.java:4247)
        at com.sun.tools.javac.main.JavaCompiler.attribute(JavaCompiler.java:1232)
        at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:871)
        at com.sun.tools.javac.main.Main.compile(Main.java:254)
        at com.sun.tools.javac.main.Main.compile(Main.java:142)
        at com.sun.tools.javac.Main.compile(Main.java:56)
        at com.sun.tools.javac.Main.main(Main.java:42)
---

This is caused by JavacParser.foldStrings returning null when allowStringFolding == false