Fix checker panic

Fixes #453

Author: antonmedv

checker/checker.go

@@ -560,6 +560,11 @@ func (v *checker) functionReturnType(node *ast.CallNode) (reflect.Type, info) {
 			fnName = name.Value
 		}
 	}
+
+	if fn == nil {
+		return v.error(node, "%v is nil; cannot call nil as function", fnName)
+	}
+
 	switch fn.Kind() {
 	case reflect.Interface:
 		return anyType, info{}

expr_test.go

@@ -2014,3 +2014,11 @@ func TestIssue432(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, float64(10), out)
 }
+
+func TestIssue453(t *testing.T) {
+	env := map[string]any{
+		"foo": nil,
+	}
+	_, err := expr.Compile(`foo()`, expr.Env(env))
+	require.Error(t, err)
+}