Avoid starting a write transaction under ShowCanCreateAgent

[Joshua (mynjj)]

See bug 646707 for the platform side, explaining why we are calling ShowCanCreateAgent after the Commit when we have 3rd party code like:

procedure ChangeUserName()  
var  
    User: Record User;  
    ShowUserQst: Label 'Show User?';  
begin  
    User.FindFirst();  
    User."Full Name" := 'New Full Name';  
    User.Modify();  
    Commit();  
    if Confirm(ShowUserQst, false) then  
        Page.RunModal(Page::"User Card", User);  
end;  

In the meantime, we ensure that no write transactions are started when computing ShowCanCreateAgent.

Fixes AB#641849