Getting this error on importing ECS in an on going project in 2018.1.0f2 and 2018.2.0f2 System.Missi

I wanted to use C# Job System and ECS in my project. I tried learning it first, it was awesome. But then in the same Unity 2018.1.0f2 in the same system, i am trying to import entities packages in my on going project, i am having these errors :

C:/Users/Default/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.burst@0.2.4-preview.20/Editor/BurstReflection.cs(132,81): error CS0121: The call is ambiguous between the following methods or properties: 'System.MissingExtensions.GetCustomAttribute(System.Reflection.MemberInfo)' and 'System.Reflection.CustomAttributeExtensions.GetCustomAttribute(System.Reflection.MemberInfo)'

I am using Scripting Runtime Version .NET 4.X Equivalent and i have .NET 4.5 in my system. I have tried to install Burst and Incremental Compiler, still no luck. I have tried Unity 2018.2.0f2 also. Right now my project is running on Android, but will soon need to port on iOS soon.

We have a production release pending with enhanced features of C# Job System and ECS.

 

What version of
- Entities package
- Incremental compiler package
- Burst package - 0.2.4p20

Do you get the error in an empty project with just the packages imported?

 

Did you able to solve this problem? I have the exact same error.

 

Looks like Firebase was causing the problem for me.
Firebase is using Unity.Tasks.dll and there are 2 in the project one for Net3.5 and one for Net4.x. The only thing I had to do was to disable the Net3.5 dll and enable the Net4.x dll.