fix(ui5-shellbar): do not duplicate popover menu items (#1456)

Author: vladitasev

packages/base/src/UI5Element.js

@@ -748,10 +748,22 @@ class UI5Element extends HTMLElement {
 				throw new Error(`"${prop}" is not a valid property name. Use a name that does not collide with DOM APIs`);
 			}
 
-			if (propData.type === "boolean" && propData.defaultValue) {
+			if (propData.type === Boolean && propData.defaultValue) {
 				throw new Error(`Cannot set a default value for property "${prop}". All booleans are false by default.`);
 			}
 
+			if (propData.type === Array) {
+				throw new Error(`Wrong type for property "${prop}". Properties cannot be of type Array - use "multiple: true" and set "type" to the single value type, such as "String", "Object", etc...`);
+			}
+
+			if (propData.type === Object && propData.defaultValue) {
+				throw new Error(`Cannot set a default value for property "${prop}". All properties of type "Object" are empty objects by default.`);
+			}
+
+			if (propData.multiple && propData.defaultValue) {
+				throw new Error(`Cannot set a default value for property "${prop}". All multiple properties are empty arrays by default.`);
+			}
+
 			Object.defineProperty(proto, prop, {
 				get() {
 					if (this._state[prop] !== undefined) {

packages/fiori/src/ShellBar.js

@@ -138,8 +138,8 @@ const metadata = {
 		},
 
 		_menuPopoverItems: {
-			type: Array,
-			defaultValue: [],
+			type: String,
+			multiple: true,
 		},
 	},
 	managedSlots: true,