added check for x for simplify_exponential_function

src/main/py/read_math_function.py

@@ -75,4 +75,6 @@ def extract_constant(function):
 
 def simplify_exponential_function(type, function):
     if type != "e":
-        return False
+        return False
+    if 'x' not in function:
+        return "Der Ausdruck muss die Variable 'x' enthalten."