Reset Clipboard in AtomEnvironment::reset

src/atom-environment.coffee

@@ -247,6 +247,8 @@ class AtomEnvironment extends Model
 
     @menu.clear()
 
+    @clipboard.reset()
+
   ###
   Section: Event Subscription
   ###

src/clipboard.coffee

@@ -14,8 +14,12 @@ clipboard = require './safe-clipboard'
 # ```
 module.exports =
 class Clipboard
-  metadata: null
-  signatureForMetadata: null
+  constructor: ->
+    @reset()
+
+  reset: ->
+    @metadata = null
+    @signatureForMetadata = null
 
   # Creates an `md5` hash of some text.
   #