use static field as ref parameter

Author: RomanSoloweow

src/SimpleStateMachine.StructuralSearch.Tests/ReplaceRuleParserTests.cs

@@ -18,7 +18,7 @@ public static class ReplaceRuleParserTests
     public static void ReplaceRuleParsingShouldBeSuccess(string replaceRule)
     {
         var rule = StructuralSearch.ParseReplaceRule(replaceRule);
-        var ruleStr = rule.ToString().ToLower();
+        var ruleStr = rule.ToString()?.ToLower();
         Assert.NotNull(rule);
         Assert.Equal(ruleStr, replaceRule.ToLower());
     }
@@ -28,7 +28,7 @@ public static void ReplaceRuleParsingShouldBeSuccess(string replaceRule)
     public static void ReplaceRuleShouldBeEqualsString(string replaceRule, string customResult)
     {
         var rule = StructuralSearch.ParseReplaceRule(replaceRule);
-        var ruleStr = rule.ToString().ToLower();
+        var ruleStr = rule.ToString()?.ToLower();
         Assert.NotNull(rule);
         Assert.Equal(ruleStr, customResult.ToLower());
     }

src/SimpleStateMachine.StructuralSearch/StructuralSearchParser.cs

@@ -51,7 +51,8 @@ public IEnumerable<FindParserResult> ApplyFindRule(ref IParsingContext context,
                 }
             }
 
-            SetFindRulesContext(ref ParsingContext.Empty);
+            IParsingContext emptyParsingContext = ParsingContext.Empty;
+            SetFindRulesContext(ref emptyParsingContext);
             return result;
         }
         public IEnumerable<FindParserResult> ApplyReplaceRule(ref IParsingContext context, IEnumerable<FindParserResult> matches)
@@ -85,8 +86,9 @@ public IEnumerable<FindParserResult> ApplyReplaceRule(ref IParsingContext contex
 
                 result.Add(match with { Placeholders = placeholders });
             }
-            
-            SetReplaceRulesContext(ref ParsingContext.Empty);
+
+            IParsingContext emptyParsingContext = ParsingContext.Empty;
+            SetReplaceRulesContext(ref emptyParsingContext);
 
             return result;
         }