added check for x for simplify_exponential_function

src/main/py/read_math_function.py

@@ -76,3 +76,5 @@ def extract_constant(function):
 def simplify_exponential_function(type, function):
     if type != "e":
         return False
+    if 'x' not in function:
+        return "Der Ausdruck muss die Variable 'x' enthalten."