you need to relook at your constructors.
The error is informing you that within the JavaScriptInterface class its looking for a constructor that takes a PApplet object. therefore you need another constructor in your class in the lines of.
JavaScriptInterface(PApplet app) {
p = app;
}
this should resolve your error but not your problem as you have not retrieved the context of the sketch, and the class needs this for its functionality.