fix(previewer): check bufnr is not nil (#3410)

Author: jam1015

lua/telescope/previewers/buffer_previewer.lua

@@ -545,9 +545,11 @@ previewers.vimgrep = defaulter(function(opts)
 
       local middle_ln = math.floor(lnum + (lnend - lnum) / 2)
       pcall(vim.api.nvim_win_set_cursor, self.state.winid, { middle_ln + 1, 0 })
-      vim.api.nvim_buf_call(bufnr, function()
-        vim.cmd "norm! zz"
-      end)
+      if bufnr ~= nil then
+        vim.api.nvim_buf_call(bufnr, function()
+          vim.cmd "norm! zz"
+        end)
+      end
     end
   end