Add a mock-based test for Editor.prototype.split and ensure it doesn't raise exceptions when not in a pane

spec/app/editor-spec.coffee

@@ -2144,3 +2144,26 @@ describe "Editor", ->
       expect(editor.buffer.path).toBe "0"
       editor.loadPreviousEditSession()
       expect(editor.buffer.path).toBe "2"
+
+  describe "split methods", ->
+    describe "when inside a pane", ->
+      fakePane = null
+      beforeEach ->
+        fakePane = { splitUp: jasmine.createSpy('splitUp') }
+        spyOn(editor, 'pane').andReturn(fakePane)
+
+      it "calls the corresponding split method on the containing pane with a copy of the editor", ->
+        editor.splitUp()
+        expect(fakePane.splitUp).toHaveBeenCalled()
+        [editorCopy] = fakePane.splitUp.argsForCall[0]
+        expect(editorCopy.serialize()).toEqual editor.serialize()
+        expect(editorCopy).not.toBe editor
+
+    describe "when not inside a pane", ->
+      it "does not split the editor, but doesn't throw an exception", ->
+        editor.splitUp()
+        editor.splitDown()
+        editor.splitLeft()
+        editor.splitRight()
+
+

src/app/editor.coffee

@@ -494,16 +494,16 @@ class Editor extends View
     @setCursorBufferPosition(fold.start)
 
   splitLeft: ->
-    @pane().splitLeft(@copy())
+    @pane()?.splitLeft(@copy())
 
   splitRight: ->
-    @pane().splitRight(@copy())
+    @pane()?.splitRight(@copy())
 
   splitUp: ->
-    @pane().splitUp(@copy())
+    @pane()?.splitUp(@copy())
 
   splitDown: ->
-    @pane().splitDown(@copy())
+    @pane()?.splitDown(@copy())
 
   pane: ->
     @parent('.pane').view()