Layer error: ImageCollection.mosaic: Invalid type. (Earth Engine)

Question

An error occurred in raster to vector conversion:

Layer 1: Layer error: ImageCollection.mosaic: Invalid type. Expected type: Image. Actual type: FeatureCollection.

Why is the Map function trying to mosaic an image collection when I have a feature collection that is also a valid input to Map.

My code link is: https://code.earthengine.google.com/f14646de49f536d7ed4b4e0d5a9ccb21

  var elevation=SRTM;
  var sorted = L8.filter(ee.Filter.eq('WRS_PATH', 147))
  .filter(ee.Filter.eq('WRS_ROW', 38))
  .filter(ee.Filter.calendarRange(2013,2013,'year'))
 .filter(ee.Filter.calendarRange(7,10,'month'))
 .filterMetadata('CLOUD_COVER', 'less_than', 10)


var ndwi1 = sorted.map(
 function(img) {

   var ndwi = img.normalizedDifference(['B3','B5']);
   var slop = ee.Terrain.slope(elevation);
   var evi1 = ndwi.expression('(thr>0.25) && (slp<20)? 1 : 0' , 

      {  'thr' : ndwi,  'slp' : slop   });


   var evi = evi1.clip(table)

   var vectors = evi.addBands(evi)
     .reduceToVectors({
      geometry: table,
      crs: evi.projection(),
      scale: 30,
      geometryType: 'polygon',
      reducer: ee.Reducer.mean(),
      maxPixels: 1e8
                     }); 

 return vectors;
 })


 print(ndwi1)
Map.addLayer(ndwi1)

Answer 1

I can't run the code because the table asset is not shared. However, I think you will need to cast ndwi1 as an ee.FeatureCollection() and then flatten() it. Mapping over the sorted image collection returns a collection of feature collections, which cannot be added as a Map layer. Replace your last two lines with this:

var ndwiFc = ee.FeatureCollection(ndwi1).flatten()
print(ndwiFc)
Map.addLayer(ndwiFc)