feat: use better approach to check SourceTree's installation

Author: Saiya

Diff for: SourceTree.alfredworkflow

@@ -163,10 +163,14 @@ class SourceTree: Workflow {
 
     emptyMessage = AlfredItem(title: "Your SourceTree Bookmark Is Empty ", subtitle: "Please add repos to SourceTree first")
 
-    guard let data = try? Data(contentsOf: Self.plistPath) else {
+    guard Self.isSourceTreeInstalled() else {
       errorMessage = AlfredItem(title: "SourceTree not installed", subtitle: "Press enter to open SourceTree homepage and download it", arg: "open \"https://sourcetreeapp.com/\"")
       return
     }
+    guard let data = try? Data(contentsOf: Self.plistPath) else {
+      errorMessage = emptyMessage
+      return
+    }
 
     do {
       let parsed = try PropertyListDecoder().decode(SourceTreePlist.self, from: data)
@@ -237,6 +241,10 @@ class SourceTree: Workflow {
     let url = FileManager.default.homeDirectoryForCurrentUser
     return url.appendingPathComponent("Library/Application Support/SourceTree/browser.plist")
   }
+
+  static func isSourceTreeInstalled() -> Bool {
+    return FileManager.default.fileExists(atPath: "/Applications/SourceTree.app")
+  }
 }
 
 // MARK: SourceTree Plist