improved error handling

app.py

@@ -16,15 +16,24 @@ def app(*argv):
     # Unpack the arguments
     seq, trg, model_name, *_ = argv
     scoring = SCORING[scoring_strategy.value]
+
+    # Initialise the output
+    out = Image(visible=False), DataFrame(visible=False)
+
+    # Validate the input
+    if 1 > len(seq):
+        raise Error("Sequence cannot be empty")
+    if 1 > len(trg):
+        raise Error("Substitutions cannot be empty")
+
     # Calculate the data based on the input parameters
     try:
         data = Data(seq, trg, model_name, scoring).calculate()
         if isinstance(data.image(), str):
-            out = Image(value=data.image(), type='filepath', visible=True), DataFrame(visible=False)
+            out = Image(value=data.image(), type='filepath', visible=True), out[1]
         else:
-            out = Image(visible=False), DataFrame(value=data.image(), visible=True)
+            out = out[0], DataFrame(value=data.image(), visible=True)
     except Exception as e:
-        out = Image(visible=False), DataFrame(visible=False)
         raise Error(str(e))
         
     return *out, DownloadButton(value=data.csv(), visible=True)