Fixed a crash that occurs when trying to add a new feed with no active accounts

Mac/MainWindow/MainWindowController.swift

@@ -227,6 +227,10 @@ class MainWindowController : NSWindowController, NSUserInterfaceValidations {
 
 			return true
 		}
+		
+		if item.action == #selector(showAddFeedWindow(_:)) {
+			return canAddNewFeed()
+		}
 
 		return true
 	}
@@ -591,6 +595,10 @@ private extension MainWindowController {
 		
 		return currentTimelineViewController?.canMarkAllAsRead() ?? false
 	}
+	
+	func canAddNewFeed() -> Bool {
+		return sidebarViewController?.canAddNewFeed() ?? false
+	}
 
 	func validateToggleRead(_ item: NSValidatedUserInterfaceItem) -> Bool {
 

Mac/MainWindow/Sidebar/SidebarViewController.swift

@@ -345,10 +345,9 @@ protocol SidebarDelegate: class {
 	// MARK: - API
 
 	func rebuildTreeAndRestoreSelection() {
-		
-		let savedAccounts = treeController.rootNode.childNodes.compactMap { $0.representedObject as? Account }
-		
+		let savedAccounts = accountNodes
 		let savedSelection = selectedNodes
+		
 		rebuildTreeAndReloadDataIfNeeded()
 		restoreSelection(to: savedSelection, sendNotificationIfChanged: true)
 		
@@ -361,6 +360,11 @@ protocol SidebarDelegate: class {
 		}
 		
 	}
+	
+	func canAddNewFeed() -> Bool {
+		return !accountNodes.isEmpty
+	}
+	
 }
 
 // MARK: - NSUserInterfaceValidations
@@ -380,6 +384,10 @@ extension SidebarViewController: NSUserInterfaceValidations {
 
 private extension SidebarViewController {
 	
+	var accountNodes: [Account] {
+		return treeController.rootNode.childNodes.compactMap { $0.representedObject as? Account }
+	}
+	
 	var selectedNodes: [Node] {
 		if let nodes = outlineView.selectedItems as? [Node] {
 			return nodes