Error popup changed to use theme colors (akvelon#223)

* error popup changed to use theme colors

* null check added

Author: yescorp

lib/src/code_field/code_field.dart

@@ -420,6 +420,7 @@ class _CodeFieldState extends State<CodeField> {
     final gutterStyle = widget.gutterStyle.copyWith(
       textStyle: lineNumberTextStyle,
       errorPopupTextStyle: widget.gutterStyle.errorPopupTextStyle ??
+          CodeTheme.of(context)?.styles['root'] ??
           textStyle.copyWith(
             fontSize: DefaultStyles.errorPopupTextSize,
             backgroundColor: DefaultStyles.backgroundColor,

lib/src/gutter/error.dart

@@ -100,9 +100,11 @@ class _GutterErrorWidgetState extends State<GutterErrorWidget> {
                     ),
                     decoration: BoxDecoration(
                       color: style.backgroundColor,
-                      border: Border.all(
-                        color: style.color!,
-                      ),
+                      border: style.color != null
+                          ? Border.all(
+                              color: style.color!,
+                            )
+                          : null,
                     ),
                     child: Column(
                       crossAxisAlignment: CrossAxisAlignment.start,