fix(ui): disable invoke button if not valid model is selected

a (hopefully) rare edge case.

Author: psychedelicious

invokeai/frontend/web/src/common/hooks/useIsReadyToInvoke.ts

@@ -18,9 +18,18 @@ const readinessSelector = createSelector(
     let isReady = true;
     const reasonsWhyNotReady: string[] = [];
 
-    if (activeTabName === 'img2img' && !initialImage) {
-      isReady = false;
-      reasonsWhyNotReady.push('No initial image selected');
+    if (activeTabName === 'img2img') {
+      if (!initialImage) {
+        isReady = false;
+        reasonsWhyNotReady.push('No initial image selected');
+      }
+    }
+
+    if (activeTabName !== 'nodes') {
+      if (!state.generation.model) {
+        isReady = false;
+        reasonsWhyNotReady.push('No main model selected');
+      }
     }
 
     const { isSuccess: mainModelsSuccessfullyLoaded } =