title: Geopandas Spatial Join - AttributeError: 'NoneType' object has no attribute 'bounds'
body: <p>I have the following geodataframes, corresponding to a regular grid:</p>
  
  <pre><code>        id   grid_id    geometry
  0       48         0    (POLYGON ((2.052457758079306 41.42493869117656...
  1       49         1    (POLYGON ((2.052470852112577 41.42403805731954...
  2       215        2    (POLYGON ((2.053641274433816 41.42584917461342...
  3       216        3    (POLYGON ((2.053654352531821 41.42494854059127...
  4       217        4    (POLYGON ((2.053667430035439 41.42404790642426...
  </code></pre>
  
  <p>and a points geodataframe:</p>
  
  <pre><code>    id_act  geometry
  0   4001    POINT (2.183026563657264 41.37459702541483)
  1   4003    POINT (2.183012216695291 41.37471411724238)
  2   4003    POINT (2.183128113845906 41.37472901746361)
  3   3002    POINT (2.182820482338962 41.37482095671629)
  4   4003    POINT (2.182945418252172 41.37482221760939)
  </code></pre>
  
  <p>I'm merging the two dataframes through a spatial join:</p>
  
  <pre><code>id_grid = gpd.sjoin(gdf, grid, how="inner", op='intersects')
  </code></pre>
  
  <p>but it returns the following <code>AttributeError</code>:</p>
  
  <pre><code>AttributeError: 'NoneType' object has no attribute 'bounds'
  </code></pre>
  
  <p>the point is that when I call the function:</p>
  
  <pre><code>grid.bounds
  </code></pre>
  
  <p>it yields:</p>
  
  <pre><code>        minx         miny       maxx         maxy
  0   2.052458    41.424038   2.053667    41.424949
  1   2.052471    41.423137   2.053681    41.424048
  2   2.053641    41.424949   2.054851    41.425859
  3   2.053654    41.424048   2.054864    41.424958
  4   2.053667    41.423147   2.054877    41.424058
  5   2.053681    41.422247   2.054890    41.423157
  </code></pre>
  
  <p>calling both <code>type(gdf.geometry[0])</code> or <code>type(grid.geometry[0])</code> gives:</p>
  
  <pre><code>shapely.geometry.point.Point
  shapely.geometry.multipolygon.MultiPolygon
  </code></pre>
  
  <p>respectively.</p>
  
  <p>Does anyone know where is the mistake?</p>