Downsample volume layer modal crashes

[philippotto]

Context

volume annotation with only one magnification (which should be downsampled via the dialog)

Current Behavior

console.js:13 Error: Layer with name undefined was not found.
    at y.getLayerByName (model.js:235)
    at y.getSegmentationTracingLayer (model.js:157)
    at ov.getVolumeMagsToDownsample (layer_settings_tab.js:684)
    at ov.render (layer_settings_tab.js:885)
    at rs (react-dom.production.min.js:182)
    at ts (react-dom.production.min.js:181)
    at $u (react-dom.production.min.js:263)
    at Fu (react-dom.production.min.js:246)
    at zu (react-dom.production.min.js:246)
    at Ou (react-dom.production.min.js:239)

Steps to Reproduce the bug

• Cannot reproduce the bug anymore / needs deeper investigation.

1. create volume annotation with only one magnification (for example, temporarily remove mags from a segmentation layer; create volume annotation with fallback; readd the mags to the dataset)
2. click the orange "downsample" button in the left sidebar

Your Environment for bug

• Browser name and version: e.g. Chrome 39
• Operating System and version: e.g. Windows 10
• Version of WebKnossos (Release or Commit):
• Specific to long-running jobs (set jobsEnabled=true in application.conf)
• Specific to webKnossos.org (set isDemoInstance=true in application.conf)

[MichaelBuessemeyer]

Fixed in #6058