remove builder on update (#3039)

api/server/handlers/stacks/create_porter_app.go

@@ -300,7 +300,11 @@ func (c *CreatePorterAppHandler) ServeHTTP(w http.ResponseWriter, r *http.Reques
 			app.BuildContext = request.BuildContext
 		}
 		if request.Builder != "" {
-			app.Builder = request.Builder
+			if request.Builder == "null" {
+				app.Builder = ""
+			} else {
+				app.Builder = request.Builder
+			}
 		}
 		if request.Buildpacks != "" {
 			if request.Buildpacks == "null" {

dashboard/src/main/home/app-dashboard/expanded-app/BuildSettingsTabStack.tsx

@@ -182,7 +182,9 @@ const BuildSettingsTabStack: React.FC<Props> = ({
         repo_name: appData.app.repo_name,
         git_branch: branch,
         build_context: folderPath,
-        builder: buildConfig.builder,
+        builder: buildView === "buildpacks"
+          ? buildConfig.builder
+          : "null",
         buildpacks:
           buildView === "buildpacks"
             ? buildConfig?.buildpacks?.join(",")