Trying to run the following script:

def f(def p) {
  echo "called with $p"
}

f('a')
f(null)

fp = this.&f

fp('b')
fp(null)

The script successfully outputs the direct call with 'a' and null and the call through the function pointer with 'b' but fails when calling with null with the following info:

java.lang.NoSuchMethodError: No such DSL method 'f' found among steps...
  at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:201)
  at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:122)
....