Fixed issue in ansi.async_alert_str() which would raise IndexError if prompt was blank. (#1188)

Author: kmvanbrunt

CHANGELOG.md

@@ -1,4 +1,6 @@
 ## 2.3.4 (TBD, 2021)
+* Bug Fixes
+  * Fixed issue in `ansi.async_alert_str()` which would raise `IndexError` if prompt was blank.
 * Enhancements
   *  Added broader exception handling when enabling clipboard functionality via `pyperclip`.
 

cmd2/ansi.py

@@ -1046,7 +1046,7 @@ def async_alert_str(*, terminal_columns: int, prompt: str, line: str, cursor_off
     """
 
     # Split the prompt lines since it can contain newline characters.
-    prompt_lines = prompt.splitlines()
+    prompt_lines = prompt.splitlines() or ['']
 
     # Calculate how many terminal lines are taken up by all prompt lines except for the last one.
     # That will be included in the input lines calculations since that is where the cursor is.