Add TravisCi configuration file

+language: dart
+dist: bionic
+addons:
+  apt:
+    packages:
+      - lib32stdc++6
+install:
+  - git clone https://github.com/flutter/flutter.git -b stable --depth 1 $HOME/flutter
+  - export PATH=$HOME/flutter/bin:$PATH
+  - flutter doctor
+  - flutter --version
+script:
+  - make analyze-ci-pdf
+  - make analyze-ci-printing
+  - make test
+  - make test-readme

@@ -117,6 +117,16 @@ analyze-printing: .pana
 
 analyze: analyze-pdf analyze-printing
 
+analyze-ci-pdf: .pana
+	@find pdf -name pubspec.yaml -exec sed -i -e 's/^dependency_overrides:/_dependency_overrides:/g' '{}' ';'
+	@pub global run pana --no-warning --source path pdf
+	@find pdf -name pubspec.yaml -exec sed -i -e 's/^_dependency_overrides:/dependency_overrides:/g' '{}' ';'
+
+analyze-ci-printing: .pana
+	@find printing -name pubspec.yaml -exec sed -i -e 's/^dependency_overrides:/_dependency_overrides:/g' '{}' ';'
+	@pub global run pana --no-warning --source path printing
+	@find printing -name pubspec.yaml -exec sed -i -e 's/^_dependency_overrides:/dependency_overrides:/g' '{}' ';'
+
 .dartfix:
 	pub global activate dartfix
 	touch $@

 # Pdf for Dart and Flutter
 
-This set of plugin allows Flutter apps to generate and print pdf files to device printer.
-This plugin works for iOS and Android.
+[![Build Status](https://travis-ci.org/DavBfr/dart_pdf.svg?branch=master)](https://travis-ci.org/DavBfr/dart_pdf)
 
-* dart pdf: <https://pub.dartlang.org/packages/pdf>
-* flutter printing: <https://pub.dartlang.org/packages/printing>
+This set of plugin allows Flutter apps to generate and print pdf files to device printer. This plugin works for iOS and Android.
+
+- dart pdf: <https://pub.dartlang.org/packages/pdf>
+- flutter printing: <https://pub.dartlang.org/packages/printing>
 
 [![Buy Me A Coffee](https://bmc-cdn.nyc3.digitaloceanspaces.com/BMC-button-images/custom_images/orange_img.png "Buy Me A Coffee")](https://www.buymeacoffee.com/JORBmbw9h "Buy Me A Coffee")

@@ -19,6 +19,10 @@ import 'dart:io';
 import 'package:markdown/markdown.dart' as md;
 
 Iterable<String> getCode(List<md.Node> nodes, [bool isCode = false]) sync* {
+  if (nodes == null) {
+    return;
+  }
+
   for (md.Node node in nodes) {
     if (node is md.Element) {
       // print(node.tag);