diff --git a/ios/Classes/SwiftFlutterHtmlToPdfPlugin.swift b/ios/Classes/SwiftFlutterHtmlToPdfPlugin.swift
index b9d51d2..2382db1 100644
--- a/ios/Classes/SwiftFlutterHtmlToPdfPlugin.swift
+++ b/ios/Classes/SwiftFlutterHtmlToPdfPlugin.swift
@@ -5,6 +5,7 @@ import WebKit
 public class SwiftFlutterHtmlToPdfPlugin: NSObject, FlutterPlugin{
     var wkWebView : WKWebView!
     var urlObservation: NSKeyValueObservation?
+    private var didFinishOnce: Bool = false
     
     public static func register(with registrar: FlutterPluginRegistrar) {
     let channel = FlutterMethodChannel(name: "flutter_html_to_pdf", binaryMessenger: registrar.messenger())
@@ -12,28 +13,72 @@ public class SwiftFlutterHtmlToPdfPlugin: NSObject, FlutterPlugin{
     registrar.addMethodCallDelegate(instance, channel: channel)
   }
     
+    private func topViewController() -> UIViewController? {
+        let window: UIWindow?
+        if #available(iOS 13.0, *) {
+            window = UIApplication.shared.connectedScenes
+                .compactMap { $0 as? UIWindowScene }
+                .first(where: { $0.activationState == .foregroundActive })?
+                .windows
+                .first(where: { $0.isKeyWindow })
+        } else {
+            window = UIApplication.shared.keyWindow ?? UIApplication.shared.delegate?.window ?? nil
+        }
+        
+        var vc = window?.rootViewController
+        while let presented = vc?.presentedViewController {
+            vc = presented
+        }
+        return vc
+    }
+    
     public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
     switch call.method {
     case "convertHtmlToPdf":
-        let args = call.arguments as? [String: Any]
-        let htmlFilePath = args!["htmlFilePath"] as? String
+        guard let args = call.arguments as? [String: Any],
+              let htmlFilePath = args["htmlFilePath"] as? String,
+              !htmlFilePath.isEmpty else {
+            result(FlutterError(code: "INVALID_ARGUMENTS",
+                                message: "Missing htmlFilePath",
+                                details: nil))
+            return
+        }
         
         // !!! this is workaround for issue with rendering PDF images on iOS !!!
-        let viewControler = UIApplication.shared.delegate?.window?!.rootViewController
-        wkWebView = WKWebView.init(frame: viewControler!.view.bounds)
+        guard let viewControler = topViewController() else {
+            result(FlutterError(code: "NO_VIEW_CONTROLLER",
+                                message: "Could not find a root view controller",
+                                details: nil))
+            return
+        }
+        
+        didFinishOnce = false
+        wkWebView = WKWebView.init(frame: viewControler.view.bounds)
         wkWebView.isHidden = true
         wkWebView.tag = 100
-        viewControler?.view.addSubview(wkWebView)
+        viewControler.view.addSubview(wkWebView)
         
-        let htmlFileContent = FileHelper.getContent(from: htmlFilePath!) // get html content from file
+        let htmlFileContent = FileHelper.getContent(from: htmlFilePath) // get html content from file
         wkWebView.loadHTMLString(htmlFileContent, baseURL: Bundle.main.bundleURL) // load html into hidden webview
         
-        urlObservation = wkWebView.observe(\.isLoading, changeHandler: { (webView, change) in
+        urlObservation = wkWebView.observe(\.isLoading, changeHandler: { [weak self] (webView, change) in
+            guard let self = self else { return }
+            if webView.isLoading { return }
+            if self.didFinishOnce { return }
+            self.didFinishOnce = true
+            
             // this is workaround for issue with loading local images
             DispatchQueue.main.asyncAfter(deadline: .now() + 1.5) {
-                let convertedFileURL = PDFCreator.create(printFormatter: self.wkWebView.viewPrintFormatter())
+                guard let wkWebView = self.wkWebView else {
+                    result(FlutterError(code: "WEBVIEW_DISPOSED",
+                                        message: "WKWebView was disposed before PDF generation finished",
+                                        details: nil))
+                    return
+                }
+                
+                let convertedFileURL = PDFCreator.create(printFormatter: wkWebView.viewPrintFormatter())
                 let convertedFilePath = convertedFileURL.absoluteString.replacingOccurrences(of: "file://", with: "") // return generated pdf path
-                if let viewWithTag = viewControler?.view.viewWithTag(100) {
+                if let viewWithTag = viewControler.view.viewWithTag(100) {
                     viewWithTag.removeFromSuperview() // remove hidden webview when pdf is generated
                     
                     // clear WKWebView cache
diff --git a/pubspec.yaml b/pubspec.yaml
index db314ef..7d9450b 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@ version: 0.7.0
 homepage: https://github.com/Afur/flutter_html_to_pdf
 
 environment:
-  sdk: '>=2.12.0 <3.0.0'
+  sdk: '>=2.12.0 <4.0.0'
   flutter: ">=1.12.0"
 
 dependencies: