fix: app crashes when invoked when there is 0 window (closes #95)

alt-tab-macos/logic/TrackedWindows.swift

@@ -22,7 +22,6 @@ class TrackedWindows {
         Spaces.currentSpaceIndex = spaces.first { $0.0 == Spaces.currentSpaceId }!.1
         filterAndAddToList(mapWindowsWithRankAndSpace(spaces))
         sortList()
-        TrackedWindows.focusedWindowIndex = TrackedWindows.moveFocusedWindowIndex(step)
     }
 
     private static func mapWindowsWithRankAndSpace(_ spaces: [(CGSSpaceID, SpaceIndex)]) -> [CGWindowID: (CGSSpaceID, SpaceIndex, WindowRank)] {

alt-tab-macos/ui/Application.swift

@@ -77,8 +77,11 @@ class Application: NSApplication, NSApplicationDelegate, NSWindowDelegate {
             isFirstSummon = false
             TrackedWindows.refreshList(step)
             if TrackedWindows.list.count == 0 {
+                appIsBeingUsed = false
+                isFirstSummon = true
                 return
             }
+            TrackedWindows.focusedWindowIndex = TrackedWindows.moveFocusedWindowIndex(step)
             let currentScreen = Screen.preferred() // fix screen between steps since it could change (e.g. mouse moved to another screen)
             if uiWorkShouldBeDone { self.thumbnailsPanel!.computeThumbnails(currentScreen); debugPrint("computeThumbnails") }
             if uiWorkShouldBeDone { self.thumbnailsPanel!.highlightCellAt(step); debugPrint("highlightCellAt") }