Throw exception when aborting/committing without a transaction

spec/app/undo-manager-spec.coffee

@@ -157,6 +157,12 @@ describe "UndoManager", ->
         undoManager.redo()
         expect(buffer.getText()).toBe '12'
 
+  describe "commit", ->
+    it "throws an exception if there is no current transaction", ->
+      expect(->
+        buffer.commit()
+      ).toThrow()
+
   describe "abort", ->
     it "does not affect the undo stack when the current transaction is empty", ->
       buffer.setText('')
@@ -167,6 +173,11 @@ describe "UndoManager", ->
       buffer.undo()
       expect(buffer.getText()).toBe ''
 
+    it "throws an exception if there is no current transaction", ->
+      expect(->
+        buffer.abort()
+      ).toThrow()
+
   describe "when a `do` operation throws an exception", ->
     it "clears the stack", ->
       spyOn(console, 'error')

src/app/undo-manager.coffee

@@ -39,16 +39,18 @@ class UndoManager
     isNewTransaction
 
   commit: ->
-    @undoHistory.push(@currentTransaction) if @currentTransaction?.length
+    unless @currentTransaction?
+      throw new Error("Trying to commit when there is no current transaction")
+
     empty = @currentTransaction.length is 0
     @undoHistory.push(@currentTransaction) unless empty
     @currentTransaction = null
     not empty
 
   abort: ->
-    @commit()
-    @undo()
-    @redoHistory.pop()
+    unless @currentTransaction?
+      throw new Error("Trying to abort when there is no current transaction")
+
     if @commit()
       @undo()
       @redoHistory.pop()