SC2Mapster

'No actor' pretty much evaluates to null, so it only tests whether the variable holds a reference, but doesn't check whether this reference is valid.

You could use "Actor Get Text" to display information about what that actor reference essentially holds.

Also, if you inspect internal implementation of this function:

actor libNtve_gf_AttachModelToUnit (unit lp_unit, string lp_model, string lp_attachPoint) {
    // Variable Declarations
    actorscope lv_scope;
    actor lv_a;

    // Automatic Variable Declarations
    // Variable Initialization

    // Implementation
    lv_scope = ActorScopeFromUnit(lp_unit);
    ActorCreate(lv_scope, "SiteHosted", lp_attachPoint, "", "");
    lv_a = libNtve_gf_ActorLastCreated();
    ActorCreate(lv_scope, "ModelHosted", (lp_model), "", "");
    lv_a = libNtve_gf_ActorLastCreated();
    return lv_a;
}

 You'll realize that it in facts creates two actors, where one is 'SiteHosted', which is some sort of link I assume (not well aware about Actors). Thus by using LastCreated action you only get the referece to ModelHosted. Although this is the essential one in fact, so it surprising if it doesn't respond to 'Destroy' message.

Anyway, you can also utilize actor cheats to get some more information about currently living actors (what could help finding out the cause), i.e.:
ActorInfoDisplay
ActorLiveDump

Could it be because I'm attaching it to a doodad instead of a unit?

You should not be capable of doing this, since it would not pass type checking, I'm assuming you're using "Attach Model To Actor". Which should actually work, because it has different implementation (if that's not the deprecated version without postfix '2'):

actor libNtve_gf_AttachModelToActor2 (actor lp_actor, string lp_model, string lp_attachPoint) {
    // Variable Declarations
    string lv_message;
    actorscope lv_scope;
    actor lv_a;

    // Automatic Variable Declarations
    // Variable Initialization
    lv_message = ("AttachModel {} ");

    // Implementation
    lv_message = ("AttachModel {} " + (lp_model) + " " + lp_attachPoint);
    ActorSendAsText(lp_actor, StringToText(lv_message));
    lv_a = libNtve_gf_ActorLastCreated();
    return lv_a;
}

 ---

Oh, wait. I just realized the implementation above uses Message to create the actor, as oposed to "ActorCreate" function. Thus you have to use "Last Created Actor by Message" to obtain valid reference in this scenario.