Fix/index 0 is out of bounds when all duration is 0 (#2)

app.py

@@ -84,11 +84,15 @@ def optimizer(
     difficulty_distribution = optimizer.difficulty_distribution.to_string().replace(
         "\n", "\n\n"
     )
-    plot_output = optimizer.find_optimal_retention(
-        learn_span=365,  # days to learn
-        max_ivl=36500,  # days
-        loss_aversion=2.5,  # forget cost is multiplied by this factor to simulate loss aversion
-    )[0]
+    try:
+        plot_output = optimizer.find_optimal_retention(
+            learn_span=365,  # days to learn
+            max_ivl=36500,  # days
+            loss_aversion=2.5,  # forget cost is multiplied by this factor to simulate loss aversion
+        )[0]
+    except:
+        print("Failed to find optimal retention")
+        optimizer.optimal_retention = 0.9
     suggested_retention_markdown = (
         f"""# Suggested Retention: `{optimizer.optimal_retention:.2f}`"""
     )