Fix crash with side inspector on El Capitan

CHANGELOG.md

@@ -15,6 +15,7 @@ develop
 ### Fixes
 
 - Fix an issue where the application crashed by the Highlight command under the condition when the find string is a invalid regular expression pattern even the regular expression is turned off.
+- Fix an issue where the application could crash on El Capitan when a side inspector is about to open.
 - Fix an issue on the text search where the single text search couldn't find the word intersects with the current selection.
 - Fix an issue where the metadata of a custom theme cannot be edited.
 - Fix an issue where the background of the line number view was drawn with wrong color when entered to the fullscreen mode.

CotEditor/Sources/WindowContentViewController.swift

@@ -254,12 +254,6 @@ final class WindowContentViewController: NSSplitViewController, TabViewControlle
     /// set visibility and tab of sidebar
     private func setSidebarShown(_ shown: Bool, index: SidebarViewController.TabIndex? = nil, animate: Bool = false) {
         
-        if let index = index {
-            self.siblings.forEach { sibling in
-                sibling.sidebarViewController!.selectedTabViewItemIndex = index.rawValue
-            }
-        }
-        
         guard NSAppKitVersionNumber >= Double(NSAppKitVersionNumber10_11) else {
             self.isSidebarShown = shown
             return
@@ -268,6 +262,12 @@ final class WindowContentViewController: NSSplitViewController, TabViewControlle
         NSAnimationContext.current().withAnimation(animate) {
             self.isSidebarShown = shown
         }
+        
+        if let index = index {
+            self.siblings.forEach { sibling in
+                sibling.sidebarViewController!.selectedTabViewItemIndex = index.rawValue
+            }
+        }
     }