Attempt to invoke virtual method 'android.content.res.Resources android.app.Activity.getResources()' on a null object reference

[RasyidMF]

When i was installing Background Service or some other service, i got this error. But somehow i fix it with my own technique...
This some error information.

W/System.err(20027): java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.app.Activity.getResources()' on a null object reference
W/System.err(20027): 	at pt.tribeiro.flutter_plugin_pdf_viewer.FlutterPluginPdfViewerPlugin.getPage(FlutterPluginPdfViewerPlugin.java:157)
W/System.err(20027): 	at pt.tribeiro.flutter_plugin_pdf_viewer.FlutterPluginPdfViewerPlugin.access$100(FlutterPluginPdfViewerPlugin.java:29)
W/System.err(20027): 	at pt.tribeiro.flutter_plugin_pdf_viewer.FlutterPluginPdfViewerPlugin$1.run(FlutterPluginPdfViewerPlugin.java:70)

When i take a look at FlutterPluginPdfViewerPlugin.java, error appear at line 157 and 158

double width = instance.activity().getResources().getDisplayMetrics().densityDpi * page.getWidth();
double height = instance.activity().getResources().getDisplayMetrics().densityDpi * page.getHeight();

I think there is some problem with activity, so i decided manually replaced it into

double width = instance.context().getResources().getDisplayMetrics().densityDpi * page.getWidth();
double height = instance.context().getResources().getDisplayMetrics().densityDpi * page.getHeight();

And it works without any error.

I don't make this as "SOLUTION" for while, i just want make sure this is safe to do.

[enveraltin]

This is fixed in #49, feel free to use my branch as a dependency if you like.