[SOLVED] Class Not Found Exception using custom cordova plugin

I'm developing a printing app that uses a custom API to access the printer via USB, so I needed a custom Cordova plugin. I started developing it, it has been a very good challenge but also very frustrating at the moment because I can't figure out why my plugin can't be used correctly. The thing is: 1.- The plugin installs correctly and lets me build the application 2.- The Javascript code runs correctly 3.- I get a runtime error that doesn't crash the app. It seems like the Java code is skipped. And I noticed there was an error in the Android Monitor.

You can find my plugin here: https://github.com/krlozadan/cordova-custom-printer-plugin

W/System.err: java.lang.ClassNotFoundException: com.duplou.cordova.plugin.customprinter.CustomPrinter
W/System.err:     at java.lang.Class.classForName(Native Method)
W/System.err:     at java.lang.Class.forName(Class.java:324)
W/System.err:     at java.lang.Class.forName(Class.java:285)
W/System.err:     at org.apache.cordova.PluginManager.instantiatePlugin(PluginManager.java:489)
W/System.err:     at org.apache.cordova.PluginManager.getPlugin(PluginManager.java:169)
W/System.err:     at org.apache.cordova.PluginManager.exec(PluginManager.java:122)
W/System.err:     at org.apache.cordova.CordovaBridge.jsExec(CordovaBridge.java:57)
W/System.err:     at org.apache.cordova.engine.SystemExposedJsApi.exec(SystemExposedJsApi.java:41)
W/System.err:     at org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
W/System.err:     at org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:53)
W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:102)
W/System.err:     at android.os.Looper.loop(Looper.java:148)
W/System.err:     at android.os.HandlerThread.run(HandlerThread.java:61)
W/System.err: Caused by: java.lang.ClassNotFoundException: Didn't find class "com.duplou.cordova.plugin.customprinter.CustomPrinter" on path: DexPathList[[zip file "/data/app/1/lib/1/base.apk!/lib/x86, /vendor/lib, /system/lib]]
W/System.err:     at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
W/System.err:     at java.lang.ClassLoader.loadClass(ClassLoader.java:511)
W/System.err:     at java.lang.ClassLoader.loadClass(ClassLoader.java:469)
W/System.err:  ... 13 more
W/System.err:  Suppressed: java.lang.ClassNotFoundException: com.duplou.cordova.plugin.customprinter.CustomPrinter
W/System.err:     at java.lang.Class.classForName(Native Method)
W/System.err:     at java.lang.BootClassLoader.findClass(ClassLoader.java:781)
W/System.err:     at java.lang.BootClassLoader.loadClass(ClassLoader.java:841)
W/System.err:     at java.lang.ClassLoader.loadClass(ClassLoader.java:504)
W/System.err:      ... 14 more
W/System.err:  Caused by: java.lang.NoClassDefFoundError: Class not found using the boot class loader; no stack trace available
I/System.out: Error adding plugin com.duplou.cordova.plugin.customprinter.CustomPrinter.
W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'void org.apache.cordova.CordovaPlugin.privateInitialize(java.lang.String, org.apache.cordova.CordovaInterface, org.apache.cordova.CordovaWebView, org.apache.cordova.CordovaPreferences)' on a null object reference
W/System.err:     at org.apache.cordova.PluginManager.getPlugin(PluginManager.java:171)
W/System.err:     at org.apache.cordova.PluginManager.exec(PluginManager.java:122)
W/System.err:     at org.apache.cordova.CordovaBridge.jsExec(CordovaBridge.java:57)
W/System.err:     at org.apache.cordova.engine.SystemExposedJsApi.exec(SystemExposedJsApi.java:41)
W/System.err:     at org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
W/System.err:     at org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:53)
W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:102)
W/System.err:     at android.os.Looper.loop(Looper.java:148)
W/System.err:     at android.os.HandlerThread.run(HandlerThread.java:61)
D/SystemWebChromeClient: file:///android_asset/www/plugins/cordova-custom-printer-plugin/www/custom-printer.js: Line 11 : Se terminó la ejecución
I/chromium: [INFO:CONSOLE(11)] "Se terminó la ejecución", source: file:///android_asset/www/plugins/cordova-custom-printer-plugin/www/custom-printer.js (11)
D/CordovaWebViewImpl: onPageFinished(file:///android_asset/www/index.html)
W/PluginManager: THREAD WARNING: exec() call to Sim.getSimInfo blocked the main thread for 53ms. Plugin should use CordovaInterface.getThreadPool().
D/SystemWebChromeClient: file:///android_asset/www/build/main.js: Line 1436 : ERROR
I/chromium: [INFO:CONSOLE(1436)] "ERROR", source: file:///android_asset/www/build/main.js (1436)
W/BindingManager: Cannot call determinedVisibility() - never saw a connection for the pid: 12139
D/EGL_emulation: eglMakeCurrent: 0xae414a40: ver 2 0 (tinfo 0xa14bfbe0)

Here's my ionic info output
global packages:
@ionic/cli-utils : 1.5.0
Cordova CLI      : 7.0.1
Ionic CLI        : 3.5.0
local packages:

@ionic/app-scripts              : 1.3.7
@ionic/cli-plugin-cordova       : 1.4.1
@ionic/cli-plugin-ionic-angular : 1.3.2
Cordova Platforms               : android 6.2.3
Ionic Framework                 : ionic-angular 3.2.1
System:

Node       : v6.10.3
OS         : macOS Sierra
Xcode      : Xcode 8.3.2 Build version 8E2002
ios-deploy : not installed
ios-sim    : not installed
npm        : 3.10.10