Disable adding a new folder if there are no active accounts

Mac/MainWindow/MainWindowController.swift

@@ -232,6 +232,10 @@ class MainWindowController : NSWindowController, NSUserInterfaceValidations {
 			return canAddNewFeed()
 		}
 		
+		if item.action == #selector(showAddFolderWindow(_:)) {
+			return canAddNewFolder()
+		}
+
 		return true
 	}
 
@@ -600,6 +604,10 @@ private extension MainWindowController {
 		return sidebarViewController?.canAddNewFeed() ?? false
 	}
 	
+	func canAddNewFolder() -> Bool {
+		return sidebarViewController?.canAddNewFolder() ?? false
+	}
+
 	func validateToggleRead(_ item: NSValidatedUserInterfaceItem) -> Bool {
 
 		let validationStatus = currentTimelineViewController?.markReadCommandStatus() ?? .canDoNothing

Mac/MainWindow/Sidebar/SidebarViewController.swift

@@ -365,6 +365,10 @@ protocol SidebarDelegate: class {
 		return !accountNodes.isEmpty
 	}
 	
+	func canAddNewFolder() -> Bool {
+		return !accountNodes.isEmpty
+	}
+	
 }
 
 // MARK: - NSUserInterfaceValidations