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(
                        "value required: instead found unexpected type " + "[" + prefixStaticType.staticType() + "]"
                    )
                );
            }
            String canonicalTypeName = semanticScope.getDecoration(userPrefixNode, PartialCanonicalTypeName.class)
                .partialCanonicalTypeName()
                + "."
                + index;
            Class<?> staticType = scriptScope.getPainlessLookup().canonicalTypeNameToType(canonicalTypeName);
            if (staticType == null) {
                semanticScope.putDecoration(userDotNode, new PartialCanonicalTypeName(canonicalTypeName));
            } else {
                if (write) {
                    throw userDotNode.createError(
                        new IllegalArgumentException(
                            "invalid assignment: "
                                + "cannot write a value to a static type ["
                                + PainlessLookupUtility.typeToCanonicalTypeName(staticType)
                                + "]"
                        )
                    );
                }
                semanticScope.putDecoration(userDotNode, new StaticType(staticType));
            }
        } else {
            Class<?> staticType = null;
            if (prefixStaticType != null) {
                String staticCanonicalTypeName = prefixStaticType.getStaticCanonicalTypeName() + "." + userDotNode.getIndex();
                staticType = scriptScope.getPainlessLookup().canonicalTypeNameToType(staticCanonicalTypeName);