Tooldef crash on NullPointer in TypeMatcher
``` tooldef
type var_data = tuple(string, string); // $name$, content.
type file_data = tuple(string, string); // suffix, path name.
list var_data vars = [];
tool string do_substitutions(string text, list var_data substs):
for subst in substs:
text = replace(text, subst[0], subst[1]);
end
return text;
end
outln(do_substitutions("$a$b$a$", vars));
````
Crashdump:
```
java.lang.NullPointerException: Cannot invoke "org.eclipse.escet.tooldef.metamodel.tooldef.types.ToolDefType.isNullable()" because "t1" is null
at org.eclipse.escet.tooldef.typechecker.TypeMatcher.computeSubType(TypeMatcher.java:118)
at org.eclipse.escet.tooldef.typechecker.TypeMatcher.computeSubType(TypeMatcher.java:153)
at org.eclipse.escet.tooldef.typechecker.ToolInvokeChecker$ToolOverload.checkTypeMatch(ToolInvokeChecker.java:481)
at org.eclipse.escet.tooldef.typechecker.ToolInvokeChecker.tcheck(ToolInvokeChecker.java:161)
at org.eclipse.escet.tooldef.typechecker.ExprsChecker.tcheck(ExprsChecker.java:636)
at org.eclipse.escet.tooldef.typechecker.ExprsChecker.tcheck(ExprsChecker.java:119)
at org.eclipse.escet.tooldef.typechecker.ToolInvokeChecker.tcheck(ToolInvokeChecker.java:151)
at org.eclipse.escet.tooldef.typechecker.StatementsChecker.tcheck(StatementsChecker.java:566)
at org.eclipse.escet.tooldef.typechecker.StatementsChecker.tcheck(StatementsChecker.java:118)
at org.eclipse.escet.tooldef.typechecker.ToolDefTypeChecker.tcheck(ToolDefTypeChecker.java:164)
at org.eclipse.escet.tooldef.typechecker.ToolDefTypeChecker.transRoot(ToolDefTypeChecker.java:112)
at org.eclipse.escet.tooldef.typechecker.ToolDefTypeChecker.transRoot(ToolDefTypeChecker.java:1)
at org.eclipse.escet.common.typechecker.TypeChecker.typeCheck(TypeChecker.java:201)
at org.eclipse.escet.setext.texteditorbase.GenericTextEditor.validateInternal(GenericTextEditor.java:689)
at org.eclipse.escet.setext.texteditorbase.GenericTextEditor.validate(GenericTextEditor.java:524)
at org.eclipse.escet.setext.texteditorbase.GenericTextEditor$DelayedValidate.run(GenericTextEditor.java:883)
```
issue