Fix find in files exception when search matches file without highlighting.

When find in files matches text in a .txt file then EditorControlService.Instance.CreateHighlighter() returns null since .txt files have no syntax highlighting.

src/AddIns/Misc/SearchAndReplace/Project/Engine/SearchManager.cs

@@ -215,7 +215,7 @@ namespace SearchAndReplace
 					var start = document.GetLocation(result.Offset).ToLocation();
 					var end = document.GetLocation(result.Offset + result.Length).ToLocation();
 					var builder = SearchResultsPad.CreateInlineBuilder(start, end, document, highlighter);
-					results.Add(new AvalonEditSearchResultMatch(fileName, start, end, result.Offset, result.Length, builder, highlighter.DefaultTextColor, result));
+					results.Add(new AvalonEditSearchResultMatch(fileName, start, end, result.Offset, result.Length, builder, highlighter, result));
 				}
 				if (results.Count > 0)
 					return new SearchedFile(fileName, results);

src/Main/Base/Project/Src/Editor/Search/SearchResultMatch.cs

@@ -110,8 +110,8 @@ namespace ICSharpCode.SharpDevelop.Editor.Search
 	{
 		ICSharpCode.AvalonEdit.Search.ISearchResult match;
 		
-		public AvalonEditSearchResultMatch(FileName fileName, Location startLocation, Location endLocation, int offset, int length, HighlightedInlineBuilder builder, HighlightingColor defaultTextColor, ICSharpCode.AvalonEdit.Search.ISearchResult match)
-			: base(fileName, startLocation, endLocation, offset, length, builder, defaultTextColor)
+		public AvalonEditSearchResultMatch(FileName fileName, Location startLocation, Location endLocation, int offset, int length, HighlightedInlineBuilder builder, ISyntaxHighlighter highlighter, ICSharpCode.AvalonEdit.Search.ISearchResult match)
+			: base(fileName, startLocation, endLocation, offset, length, builder, GetDefaultHighlightTextColor(highlighter))
 		{
 			this.match = match;
 		}
@@ -120,6 +120,14 @@ namespace ICSharpCode.SharpDevelop.Editor.Search
 		{
 			return match.ReplaceWith(pattern);
 		}
+		
+		static HighlightingColor GetDefaultHighlightTextColor(ISyntaxHighlighter highlighter)
+		{
+			if (highlighter != null) {
+				return highlighter.DefaultTextColor;
+			}
+			return null;
+		}
 	}
 	
 	public class SearchedFile