I got an unexpected internal NPE when playing with this script.

```
POST _scripts/painless/_execute
{
  "script": {
    "source": """
    return /[^\w]+/.split('abc   def higkln').toArray(String[]::new)
    """
  }
}
```
```
{
  "error" : {
    "root_cause" : [
      {
        "type" : "script_exception",
        "reason" : "compile error",
        "script_stack" : [ ],
        "script" : "\n    return /[^\\w]+/.split('abc   def higkln').toArray(String[]::new)\n    ",
        "lang" : "painless"
      }
    ],
    "type" : "script_exception",
    "reason" : "compile error",
    "script_stack" : [ ],
    "script" : "\n    return /[^\\w]+/.split('abc   def higkln').toArray(String[]::new)\n    ",
    "lang" : "painless",
    "caused_by" : {
      "type" : "null_pointer_exception",
      "reason" : "Cannot read field \"annotations\" because the return value of \"org.elasticsearch.painless.lookup.PainlessLookup.lookupPainlessClass(java.lang.Class)\" is null"
    }
  },
  "status" : 400
}

```
modules/lang-painless/src/main/java/org/elasticsearch/painless/phase/DefaultSemanticAnalysisPhase.java
new IllegalArgumentException(
                                        "field [" + prefixStaticType.getStaticCanonicalTypeName() + ", " + index + "] not found"
                                    )
                                );
                            }
                        }
                    } else {
                        if (write && Modifier.isFinal(field.javaField().getModifiers())) {
                            throw userDotNode.createError(
                                new IllegalArgumentException(
                                    "invalid assignment: cannot assign a value to read-only field [" + field.javaField().getName() + "]"
                                )
                            );
                        }
                        semanticScope.putDecoration(userDotNode, new StandardPainlessField(field));
                        valueType = field.typeParameter();
                    }
                }
                semanticScope.putDecoration(userDotNode, new ValueType(valueType));
                if (userDotNode.isNullSafe()) {
                    if (write) {
                        throw userDotNode.createError(
                            new IllegalArgumentException("invalid assignment: cannot assign a value to a null safe operation [?.]")
                        );
                    }
                    if (valueType.isPrimitive()) {
                        throw new IllegalArgumentException("Result of null safe operator must be nullable");
                    }
                }
            }