How to reproduce:
1. Open the attached project "case_1125286-test-demo_1"
2. In the Test Runner Window right-click the test "CantTouchThis" and select "Open source code"

Expected result: the associated test script is opened in the source code editor
Actual result: error or warning (depending on Unity version) is given in Console Window and source code is not opened

Reproducible with: 2017.4.20f2, 2018.3.6f1, 2019.1.0b4, 2019.2.0a6

\[...]

Reproduced in 2019.2 test-framework 1.1.9
Gives the following errors:

MethodDefinition cannot be null. Check if any method was found by name in its declaring type TypeDefinition.
UnityEditor.GenericMenu:CatchMenu(Object, String\[], Int32) (at C:/buildslave/unity/build/Editor/Mono/GUI/GenericMenu.cs:119)

Failed to open test method source code in external editor. Inconsistent filename and yield return operator in target method.
UnityEditor.GenericMenu:CatchMenu(Object, String\[], Int32) (at C:/buildslave/unity/build/Editor/Mono/GUI/GenericMenu.cs:119)