patch 8.1.2076: crash when trying to put a terminal in a popup window

Problem:    Crash when trying to put a terminal buffer in a popup window.
Solution:   Check for NULL buffer.  Do not allow putting a terminal in a popup
            window.

runtime/doc/popup.txt

@@ -1,4 +1,4 @@
-*popup.txt*  For Vim version 8.1.  Last change: 2019 Sep 08
+*popup.txt*  For Vim version 8.1.  Last change: 2019 Sep 25
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -137,9 +137,7 @@ Options can be set on the window with `setwinvar()`, e.g.: >
 	call setwinvar(winid, '&wrap', 0)
 And options can be set on the buffer with `setbufvar()`, e.g.: >
 	call setbufvar(winbufnr(winid), '&filetype', 'java')
-Note that this does not trigger autocommands.  Use `win_execute()` if you do
-need them.
-
+You can also use `win_execute()` with a ":setlocal" command.
 
 
 ==============================================================================
@@ -540,6 +538,10 @@ properties.  It is in one of four forms:
 			dictionary with a "col" entry, see below:
 			|popup-props|.
 
+If you want to create a new buffer yourself use |bufadd()| and pass the buffer
+number to popup_create().
+It is not possible to use the buffer of a terminal window. *E278*
+
 The second argument of |popup_create()| is a dictionary with options:
 	line		Screen line where to position the popup.  Can use a
 			number or "cursor", "cursor+1" or "cursor-1" to use