Not sure of input parameters for Groovy call: Ambiguous method overloading - grails

Question

I'm still new to groovy/grails, but I can't figure what's wrong with the following syntax when I try to call the mail plugin for Grails 3.0.9:
class Email {
String subject;
List<String> toEmail;
List<String> ccEmail;
List<String> bccEmail;
String body;
}
Email email = new Email(toEmail: ["test#mail.com"], body: "Hi", subject: "Yo")
mailService.sendMail {
to email.toEmail.toArray()
from "no-reply#fake.com"
cc email.ccEmail?.toArray()
bcc email.bccEmail?.toArray()
subject email.subject
body email.body
}
The exception below is being thrown at the cc parameter line when I don't have anyone to carbon copy. However, I would have thought that the elvis operator would have just returned null and all would have been well. What am I missing? I feel like perhaps I shouldn't be setting the cc and bcc fields at all if there is no data present but I'm not sure why this would cause an issue (maybe it's internal to the plugin) and how I could code around this edge case (build up the arguments separately and pass them in as a variable?)
groovy.lang.GroovyRuntimeException: Ambiguous method overloading for method org.springframework.util.Assert#notEmpty.
Cannot resolve which method to invoke for [null, class java.lang.String] due to overlapping prototypes between:
[interface java.util.Collection, class java.lang.String]
[interface java.util.Map, class java.lang.String]
at groovy.lang.MetaClassImpl.chooseMostSpecificParams(MetaClassImpl.java:3241) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaClassImpl.chooseMethodInternal(MetaClassImpl.java:3194) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaClassImpl.chooseMethod(MetaClassImpl.java:3137) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaClassImpl.pickStaticMethod(MetaClassImpl.java:1508) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaClassImpl.retrieveStaticMethod(MetaClassImpl.java:1404) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaClassImpl.createStaticSite(MetaClassImpl.java:3383) ~[groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.CallSiteArray.createCallStaticSite(CallSiteArray.java:77) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.CallSiteArray.createCallSite(CallSiteArray.java:162) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:133) [groovy-2.4.5.jar:2.4.5]
at grails.plugins.mail.MailMessageBuilder.cc(MailMessageBuilder.groovy:214) ~[mail-2.0.0.RC4.jar:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_51]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_51]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_51]
at java.lang.reflect.Method.invoke(Method.java:497) ~[na:1.8.0_51]
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:325) ~[groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:384) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1019) ~[groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.callCurrent(PogoMetaClassSite.java:69) ~[groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallCurrent(CallSiteArray.java:52) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:154) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:166) [groovy-2.4.5.jar:2.4.5]
at com.doctorsorders.email.EmailService$_sendEmail_closure1.doCall(EmailService.groovy:19) ~[main/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_51]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_51]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_51]
at java.lang.reflect.Method.invoke(Method.java:497) ~[na:1.8.0_51]
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:325) ~[groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:294) ~[groovy-2.4.5.jar:2.4.5]
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1019) ~[groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:42) ~[groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113) [groovy-2.4.5.jar:2.4.5]
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125) [groovy-2.4.5.jar:2.4.5]

Answer 1

Initialize your lists
class Email {
String subject
List<String> toEmail = []
List<String> ccEmail = []
List<String> bccEmail = []
String body
}

Answer 2

I wasn't able to get the right answer from the community, but, to be fair, I think a lot of the finnicky behavior is attributable to the way the mail plugin was written... or maybe it was the way I was using it.
I didn't realize it at the time, but I'm passing a closure to this plugin which contains the series of statements used to initialize the email arguments.
Having a statement within the closure like this:
bcc email.bccEmail?.toArray()
breaks the plugin when no e-mails are specified because the "bcc" var is created and the implementation of this plugin now expects a value. I coded around this by implementing my closure the following way:
Closure mailServiceArgs = {
to email.toEmail.toArray()
from fromEmail
subject email.subject
body email.body
if(email.ccEmail) cc email.ccEmail.toArray()
if(email.bccEmail) bcc email.bccEmail.toArray()
}
This brings up a couple of questions however, because I'm writing my closure with an if statement that is conditional based on the state of a variable (email.ccEmail) which is defined within the scope of my application. I assume this value is passed by reference so at the time the closure is invoked, the value has remained unchanged, but honestly I'm not sure how this works and will research that as well.
There's probably a 'prettier' or more elegant way to do this rather than using an 'if' statement as well.