Reproduction steps:
1. Open the attached project ("1230182.zip")
2. Open "SampleScene"
3. Select "Terrain" in the Hierarchy
4. Open the Paint Details tab in the Terrain Component
5. Click "Edit Details..." and select "Add Detail Mesh"
6. Drag the "ToBeDetailMeshGO" to the "Detail" field and click "Add" button

Reproducible with: 2017.4.38f1, 2018.4.19f1, 2019.3.7f1, 2020.1.0b3, 2020.2.0a5

First lines of the stacktraces:
at (wrapper managed-to-native) UnityEngine.TerrainData.set_detailPrototypes (UnityEngine.TerrainData,UnityEngine.DetailPrototype[]) [0x00014] in <f7066d1ad92d45278e45462c4d1030bc>:0
at UnityEditor.DetailMeshWizard.DoApply () [0x0016e] in <89faf899e9c14826b38ffd64a0c1974d>:0
at UnityEditor.DetailMeshWizard.OnWizardCreate () [0x00002] in <89faf899e9c14826b38ffd64a0c1974d>:0
at (wrapper runtime-invoke) object.runtime_invoke_void__this__ (object,intptr,intptr,intptr) [0x0004f] in <fb001e01371b4adca20013e0ac763896>:0

0x00007FF6B2809F3E (Unity) std::vector<Vector3f,std::allocator<Vector3f> >::_Assign_range<StrideIterator<Vector3f> >
0x00007FF6B2817490 (Unity) DetailDatabase::RefreshPrototypesStep1
0x00007FF6B28167FE (Unity) DetailDatabase::RefreshPrototypes
0x00007FF6B281E9D0 (Unity) DetailDatabase::SetDetailPrototypes
0x00007FF6B2802996 (Unity) TerrainDataScriptingInterface::SetDetailPrototypes

Note:
- The GameObject added as the Terrain Detail Mesh contains a Mesh Filter with a mesh that doesn't have normals, a Mesh Renderer and a default shader
- There's always an assertion failure before the crash: "Assertion failed on expression: 'm_Stride != 0 && "Stide must not be zero"'"