prevent weird null reference exception in error-handling

docs/release-notes.md

@@ -1,6 +1,10 @@
 ← [README](README.md)
 
 # Release notes
+## Upcoming release
+* For mod authors:
+  * Fixed rare `NullReferenceException` in SMAPI's error-handling.
+
 ## 3.12.2
 Released 05 August 2021 for Stardew Valley 1.5.4 or later.
 

src/SMAPI.Internal/ExceptionExtensions.cs

@@ -12,6 +12,8 @@ namespace StardewModdingAPI.Internal
         /// <summary>Get a string representation of an exception suitable for writing to the error log.</summary>
         /// <param name="exception">The error to summarize.</param>
         public static string GetLogSummary(this Exception exception)
+        {
+            try
             {
                 switch (exception)
                 {
@@ -25,7 +27,12 @@ namespace StardewModdingAPI.Internal
                         return summary;
 
                     default:
-                    return exception.ToString();
+                        return exception?.ToString() ?? $"<null exception>\n{Environment.StackTrace}";
+                }
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidOperationException($"Failed handling {exception?.GetType().FullName} (original message: {exception?.Message})", ex);
             }
         }