Generating heatmap of all cells

Dear all,
I would like to generate a heatmap of all my cells for specific genes regardless of their clusters. for this I created a new column with the name of my cells and planned to use the parameter number of categories in the heatmap function to enforce certain number of clusters but unfortunately I got the following error:

TypeError: can only concatenate str (not “float”) to str

I tried to avoid this problem by changing by dtype to category but then I can’t enforce certain number of clusters and this will be a mess! How can I move on? below you can find what I have done! Thank you!

adata.obs['cell_names']=adata.obs_names.astype(str)
# this didn't work ! so I tried this
adata.obs['cell_names']=adata.obs_names.astype("category") 
# this result in a very messy heatmap and using dendogram takes too much computing time
sc.pl.heatmap(adata,var_names=["Adgre1","Ccr2"],groupby="cell_names",num_categories=10,dendrogram=True)

If you just want to plot the genes regardless of clustering, just use groupby=None.

sc.pl.heatmap(adata,var_names=["Adgre1","Ccr2"],groupby=None)

If you want to group the genes by a numerical value for example n_genes then you use num_categories.