Don't crash when an implicit declaration is of an error type

CHANGELOG.md

@@ -16,6 +16,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ### Changed
 
 - Commands are now better at checking to see if the first word is a keyword (e.g. `return`) or a word that just _begins_ with a keyword (`returnToMenu`).
+- Fixed an error where the compiler would crash if an error expression resulted in the implicit declaration of a variable (for example, `<<set $x = >>`).
 
 ### Removed
 

Tests/TestCases/ParseFailures/SetStatements-MustHaveValues.yarn

@@ -0,0 +1,4 @@
+title: Start
+---
+<<set $x = >>
+===

YarnSpinner.Compiler/Compiler.cs

@@ -312,6 +312,13 @@ public static CompilationResult Compile(CompilationJob compilationJob)
             {
                 decl.Type = TypeChecker.ITypeExtensions.Substitute(decl.Type, typeSolution);
 
+                // If this value is of an error type, don't attempt to create a
+                // value for it.
+                if (decl.Type == Types.Error)
+                {
+                    continue;
+                }
+
                 // If this was an implicit declaration, then we didn't have an
                 // initial value to use. Instead, set its default value to one
                 // provided by the type.