onMethodCall can be passed a null receiver

[jglick]

The Javadoc for GroovyInterceptor.onMethodCall should note that receiver may be null, contrary to your expectations, and add a @CheckForNull annotation. This seems to happen when Checker.checkedComparison is called on a script line such as var == … when var is currently null (stretching the definition of “method call” a bit).

[kohsuke]

Fixed toward 1.3 by not routing such calls to the interceptor.