Choropleth layer (SQ) vanishes on revisit

[nesnoj]

Reproduce:

• go to step 2, activate an arbitrary choropleth layer
• go to step 4
• go back to step 2, choropleth is gone

Expected result: choropleth is shown again

(result choropleths in step 5 is reset on revisit but this is ok)

[nesnoj]

Solution already implemented in https://github.com/rl-institut/digiplan/pull/392