How to get dummy variables for both train and test set?

I want to create dummy variables of categorical fields for both train & test set and then train classifier only on the features common in both train & test set. I am running below code for creating dummy variables in both datasets but getting TypeError.

I typed below in one cell of Jupyter notebook

def get_features(train, test):
trainval = list(train.columns.values) # list train features
testval = list(test.columns.values) # list test features
features = list(set(trainval) & set(testval)) # check which features are in common (remove the outcome column)
features.remove('Id') # remove non-useful id column
return features

def process_features(train,test):
tables=[test,train]
for table in tables:
    table['SoldDt']= table[['MoSold','YrSold']].apply(lambda x : '{}-{}'.format(x[0],x[1]), axis=1)
    table['YearBuilt']= pd.to_datetime(table.YearBuilt,format="%Y")
    table['YearRemodAdd']= pd.to_datetime(table.YearRemodAdd,format="%Y")
    table['SoldDt']= pd.to_datetime(table.SoldDt,format="%m-%Y")
    table.GarageYrBlt.fillna(1,inplace=True)
    table.GarageYrBlt=table.GarageYrBlt.apply(int)
    table.GarageYrBlt.replace(1,'NaT',inplace=True)
    table['GarageYrBlt']= pd.to_datetime(table.GarageYrBlt,format="%Y")
    del table['MoSold']
    del table['YrSold']
    table['MSSubClass']=table['MSSubClass'].apply(str)
    table['OverallQual']=table['OverallQual'].apply(str)
    table['OverallCond']=table['OverallCond'].apply(str)
    table.Alley.fillna("NotAvl",inplace=True)
    table.BsmtQual.fillna("NB",inplace=True)
    table.BsmtCond.fillna("NB",inplace=True)
    table.BsmtExposure.fillna("NB",inplace=True)
    table.BsmtFinType1.fillna("NB",inplace=True)
    table.BsmtFinType2.fillna("NB",inplace=True)
    table.FireplaceQu.fillna("NF",inplace=True)
    table.GarageType.fillna("NG",inplace=True)
    table.GarageFinish.fillna("NG",inplace=True)
    table.GarageQual.fillna("NG",inplace=True)
    table.GarageCond.fillna("NG",inplace=True)
    table.PoolQC.fillna("NP",inplace=True)
    table.Fence.fillna("NFe",inplace=True)
    table.MiscFeature.fillna("NotAvl",inplace=True)
    table.LotFrontage.fillna(0,inplace=True)

    table=table.dropna(inplace=True)
    table=pd.get_dummies(table)

features = get_features(train,test)
return train,test,features

Then, I call the function in a different cell

train = pd.read_csv('/mnt/disk2/Data/HousePrices/train.csv')
test = pd.read_csv('/mnt/disk2/Data/HousePrices/test.csv')
train,test,features = process_features(train,test)

I am getting below error

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-17-b2727d6cdc63> in <module>()
  1 train = pd.read_csv('/mnt/disk2/Data/HousePrices/train.csv')
  2 test = pd.read_csv('/mnt/disk2/Data/HousePrices/test.csv')
----> 3 train,test,features = process_features(train,test)

<ipython-input-16-dc47e5e9f9b6> in process_features(train, test)
 40 
 41         table=table.dropna(inplace=True)
---> 42         table=pd.get_dummies(table)
 43 
 44     print ("Getting features...")

/usr/local/lib/python3.5/dist-packages/pandas/core/reshape.py in     get_dummies(data, prefix, prefix_sep, dummy_na, columns, sparse, drop_first)
   1102     else:
   1103     result = _get_dummies_1d(data, prefix, prefix_sep, dummy_na,
-> 1104                                  sparse=sparse, drop_first=drop_first)
   1105     return result
   1106 

/usr/local/lib/python3.5/dist-packages/pandas/core/reshape.py in _get_dummies_1d(data, prefix, prefix_sep, dummy_na, sparse, drop_first)
   1123     # if all NaN
   1124     if not dummy_na and len(levels) == 0:
-> 1125         return get_empty_Frame(data, sparse)
   1126 
   1127     codes = codes.copy()

/usr/local/lib/python3.5/dist-packages/pandas/core/reshape.py in get_empty_Frame(data, sparse)
   1115             index = data.index
   1116         else:
-> 1117             index = np.arange(len(data))
   1118         if not sparse:
   1119             return DataFrame(index=index)

TypeError: object of type 'NoneType' has no len()

On this line

table=table.dropna(inplace=True)

dropna returned None because the docs state

inplace : boolean, default False
    If True, do operation inplace and return None.

but then you tried to pass that None value to get_dummies().