Reproduction steps:
1. Open the attached project "inheritVelocity_rigidbody.zip";
2. Open the scene "test";
3. Click on ParticleSystem_test -> ParticleSystem -> Inherit Velocity;
4. Change the setting "Multiplier" and observe the Console window;
5. Click on ParticleSystem|_test -> Rigidbody2D ->Body Type;
6. Change the setting from Dynamic to Kinematic or Static and observe the Console window;

Actual result: the errors "NullReferenceException: Object reference not set to an instance of an object" and "Unexpected top level layout group! Missing GUILayout.EndScrollView/EndVertical/EndHorizontal?" show up.

Expected result: the errors should not show up. A note by the Inherit Velocity "Inherit velocity requires interpolation enabled on the rigidbody to function correctly" should show up.

Note: the errors appear when Rigidbody2D Body Type is set to Dynamic and ParticleSystem Inherit Velocity Multiplier is set to more or less than 0. With other Body Types, everything works correctly. Also, with Rigidbody component, the errors do not show up.

Reproduced with: 5.5.0b1, 5.5.0f1, 5.6.0b6
Not reproducible: 5.5.0a1
Regression introduced in 5.5.0a3

Could not be tested with 5.4 versions because in there Rigidobody2D does not have Body Type setting.