catch crash with cobinhood

fixes #959

freqtrade/freqtradebot.py

@@ -277,11 +277,14 @@ class FreqtradeBot(object):
             return None
 
         min_stake_amounts = []
-        if 'cost' in market['limits'] and 'min' in market['limits']['cost']:
+        limits = market['limits']
-            min_stake_amounts.append(market['limits']['cost']['min'])
+        if ('cost' in limits and 'min' in limits['cost']
+                and limits['cost']['min'] is not None):
+            min_stake_amounts.append(limits['cost']['min'])
 
-        if 'amount' in market['limits'] and 'min' in market['limits']['amount']:
+        if ('amount' in limits and 'min' in limits['amount']
-            min_stake_amounts.append(market['limits']['amount']['min'] * price)
+                and limits['amount']['min'] is not None):
+            min_stake_amounts.append(limits['amount']['min'] * price)
 
         if not min_stake_amounts:
             return None

freqtrade/tests/test_freqtradebot.py

@@ -348,6 +348,34 @@ def test_get_min_pair_stake_amount(mocker, default_conf) -> None:
     result = freqtrade._get_min_pair_stake_amount('ETH/BTC', 1)
     assert result is None
 
+    # no cost Min
+    mocker.patch(
+        'freqtrade.exchange.Exchange.get_markets',
+        MagicMock(return_value=[{
+            'symbol': 'ETH/BTC',
+            'limits': {
+                'cost': {"min": None},
+                'amount': {}
+            }
+        }])
+    )
+    result = freqtrade._get_min_pair_stake_amount('ETH/BTC', 1)
+    assert result is None
+
+    # no amount Min
+    mocker.patch(
+        'freqtrade.exchange.Exchange.get_markets',
+        MagicMock(return_value=[{
+            'symbol': 'ETH/BTC',
+            'limits': {
+                'cost': {},
+                'amount': {"min": None}
+            }
+        }])
+    )
+    result = freqtrade._get_min_pair_stake_amount('ETH/BTC', 1)
+    assert result is None
+
     # empty 'cost'/'amount' section
     mocker.patch(
         'freqtrade.exchange.Exchange.get_markets',