Eliminate invalid calls to selectActiveBrickPipelineUIState

[twschiller]

Context

• selectActiveBrickPipelineUIState is called by useAddBrick even when there's no active mod component because the AddBrickModal content is always mounted
• That currently generates a console warning on each re-render:

  pixiebrix-extension/src/pageEditor/store/editor/editorSelectors/editorPipelineSelectors.ts

  Line 26 in d25e4c0

  export function selectActiveBrickPipelineUIState({

Implementation Sketch

• Make calling selectActiveBrickPipelineUIState without an active mod component an error
• Refactor AddBrickModal to conditionally render its content. Might alternatively be able to refactor useAddBrick to use async thunks instead