UMG Animation MPC track cannot preview

The reason should be in FMaterialShader::GetParameterCollectionBuffer , Scene is nullptr and cached value GDefaultMaterialParameterCollectionInstances ** doesn't update.

1. Create a MPC(material parameter collection) and add a vector parameter
2. Create a UI Material, add MPC node and connect to emissive port
3. Create a UMG, add a image widget, select the UI Material as image brush
4. Add a UMG animation track, add some keys to the MPC vector section
5. Drag time label in the sequence

result: the image brush doesn't change in UMG editor preview

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