[blockly] Fix warning when show code button is clicked twice (#2316)

Avoid an error popup when clicking the show code button when already
in code preview mode.

Signed-off-by: Jimmy Tanagra <jcode@tanagra.id.au>

bundles/org.openhab.ui/web/src/pages/settings/rules/script/script-edit.vue

@@ -415,6 +415,8 @@ export default {
       this.$refs.blocklyEditor.showHideLabels(this.blocklyShowLabels)
     },
     showBlocklyCode () {
+      if (this.blocklyCodePreview) return
+
       try {
         this.currentModule.configuration.blockSource = this.$refs.blocklyEditor.getBlocks()
         this.script = this.$refs.blocklyEditor.getCode()