formula_creator: Remove tap accessor

Library/Homebrew/dev-cmd/create.rb

@@ -176,10 +176,11 @@ module Homebrew
       print "Formula name [#{stem}]: "
       fc.name = __gets || stem
     end
-    raise TapUnavailableError, fc.tap.name unless fc.tap.installed?
 
     fc.url = args.named.first
 
+    fc.verify
+
     # Check for disallowed formula, or names that shadow aliases,
     # unless --force is specified.
     unless args.force?

Library/Homebrew/formula_creator.rb

@@ -9,7 +9,6 @@ module Homebrew
   #
   # @api private
   class FormulaCreator
-    attr_reader :tap
     attr_accessor :name
 
     sig {
@@ -26,6 +25,11 @@ module Homebrew
       @head = head
     end
 
+    sig { void }
+    def verify
+      raise TapUnavailableError, @tap.name unless @tap.installed?
+    end
+
     def url=(url)
       @url = url
       path = Pathname.new(url)