Ensure pane is added to container before invoking onDidAddPane listeners

Fixes #12654

spec/pane-container-spec.coffee

@@ -220,7 +220,9 @@ describe "PaneContainer", ->
     it "invokes the given callback when panes are added", ->
       container = new PaneContainer(params)
       events = []
-      container.onDidAddPane (event) -> events.push(event)
+      container.onDidAddPane (event) ->
+        expect(event.pane in container.getPanes()).toBe true
+        events.push(event)
 
       pane1 = container.getActivePane()
       pane2 = pane1.splitRight()

src/pane-axis.coffee

@@ -75,12 +75,10 @@ class PaneAxis extends Model
     @onDidChangeFlexScale(fn)
 
   addChild: (child, index=@children.length) ->
+    @children.splice(index, 0, child)
     child.setParent(this)
     child.setContainer(@container)
-
     @subscribeToChild(child)
-
-    @children.splice(index, 0, child)
     @emitter.emit 'did-add-child', {child, index}
 
   adjustFlexScale: ->