fix(tabs): do not init w/ tab that is hidden or disabled

Closes #6226

ionic/components/tabs/tabs.ts

@@ -287,9 +287,30 @@ export class Tabs extends Ion {
    * @private
    */
   ngAfterContentInit() {
+    let preloadTabs = (isBlank(this.preloadTabs) ? this._config.getBoolean('preloadTabs') : isTrueProperty(this.preloadTabs));
+
+    // get the selected index
     let selectedIndex = this.selectedIndex ? parseInt(this.selectedIndex, 10) : 0;
 
-    let preloadTabs = (isBlank(this.preloadTabs) ? this._config.getBoolean('preloadTabs') : isTrueProperty(this.preloadTabs));
+    // ensure the selectedIndex isn't a hidden or disabled tab
+    // also find the first available index incase we need it later
+    let availableIndex = -1;
+    this._tabs.forEach((tab, index) => {
+      if (tab.enabled && tab.show && availableIndex < 0) {
+        // we know this tab index is safe to show
+        availableIndex = index;
+      }
+
+      if (index === selectedIndex && (!tab.enabled || !tab.show)) {
+        // the selectedIndex is not safe to show
+        selectedIndex = -1;
+      }
+    });
+    if (selectedIndex < 0) {
+      // the selected index wasn't safe to show
+      // instead use an available index found to be safe to show
+      selectedIndex = availableIndex;
+    }
 
     this._tabs.forEach((tab, index) => {
       if (index === selectedIndex) {