public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    switch call.method {
    case "isPassLibraryAvailable":
        isPassLibaryAvailable(result)
    case "canAddPasses":
        canAddPasses(result)
    case "addPasses":
        addPassesWithoutFlow(call, result)
    case "addPassesViewController":
        addPassFlow(call, result)
    case "addPassViewController":
        addPassFlow(call, result)
    case "getPasses":
        getPasses(result)
    case "containsPass":
        containsPass(call, result)
    default:
        result(FlutterMethodNotImplemented)
    }
}

case "addPassesViewController":
-    addPassFlow(call, result)
+    addPassesFlow(call, result)
case "addPassViewController":
    addPassFlow(call, result)

Corrected switch (for clarity)
public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
   switch call.method {
   case "isPassLibraryAvailable":
       isPassLibaryAvailable(result)
   case "canAddPasses":
       canAddPasses(result)
   case "addPasses":
       addPassesWithoutFlow(call, result)
   case "addPassesViewController":
       addPassesFlow(call, result)
   case "addPassViewController":
       addPassFlow(call, result)
   case "getPasses":
       getPasses(result)
   case "containsPass":
       containsPass(call, result)
   default:
       result(FlutterMethodNotImplemented)
   }
}