Fix #1810: ArgumentNullException in DecompilerTextView.ShowText

ILSpy/TextView/DecompilerTextView.cs

@@ -567,6 +567,7 @@ namespace ICSharpCode.ILSpy.TextView
 				this.nextDecompilationRun.TaskCompletionSource.TrySetCanceled();
 				this.nextDecompilationRun = null;
 			}
+			if (nodes != null && string.IsNullOrEmpty(textOutput.Title))
 				textOutput.Title = string.Join(", ", nodes.Select(n => n.ToString()));
 			ShowOutput(textOutput, highlighting);
 			decompiledNodes = nodes;