Resolved: The type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?) (Error: CS0246)

Introduction:
There is common error in xamarin.forms project when you trying to build solution for android or iOS.

"Error: The type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?) (CS0246)"
Description:
In general, you will get above error in two files:
1. Android project: MainActivity class at code line (LoadApplication(new App());. 
2. iOS project: AppDelegate class at code line LoadApplication(new App());
Solution:
If you got error like "A type or namespace that is used in the program was not found". You might have forgotten/missed to reference the assembly that contains the type, or you might not have added the required using directive. Or, there might be an issue with the assembly you are trying to reference.
For Example,

In above description, we are referring to "new App()" class object which is comes from PCL project. So we have to check valid reference of PCL project in both platform specific projects (i.e android & iOS), delete invalid-reference and put them back in to resolve the error.
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