message: fix(protocol-designer): fix bug with null distribute step
  creating a Distribute, cancelling it, and opening it again caused a whitescreen bug
diff: @@ -58,7 +58,7 @@ export const maxDispenseWellVolume = (fields: HydratedFormData): ?FormWarning =>
  export const minDisposalVolume = (fields: HydratedFormData): ?FormWarning => {
  const {aspirate_disposalVol_checkbox, aspirate_disposalVol_volume, pipette} = fields
  - const pipetteSpecs = getPipetteNameSpecs(pipette.model)
  + const pipetteSpecs = pipette && getPipetteNameSpecs(pipette.model)
  if (!pipette || !pipetteSpecs) return null
  const isUnselected = !aspirate_disposalVol_checkbox || !aspirate_disposalVol_volume
  if (isUnselected) return FORM_WARNINGS.BELOW_MIN_DISPOSAL_VOLUME
  